How to add BigBird to 🤗 Transformers?
=====================================

Mentor: [Patrick](https://github.com/patrickvonplaten)

Begin: 15.02.2020

Estimated End: 12.03.2020

Adding a new model is often difficult and requires an in-depth knowledge
of the 🤗 Transformers library and ideally also of the model's original
repository. At Hugging Face, we are trying to empower the community more
and more to add models independently. 

The following sections explain in detail how to add BigBird 
to Transformers. You will work closely with Patrick to
integrate BigBird into Transformers. By doing so, you will both gain a 
theoretical and deep practical understanding of BigBird. 
But more importantly, you will have made a major
open-source contribution to Transformers. Along the way, you will:

-   get insights into open-source best practices
-   understand the design principles of one of the most popular NLP
    libraries
-   learn how to do efficiently test large NLP models
-   learn how to integrate Python utilities like `black`, `isort`,
    `make fix-copies` into a library to always ensure clean and readable
    code

To start, let's try to get a general overview of the Transformers
library.

General overview of 🤗 Transformers
----------------------------------

First, you should get a general overview of 🤗 Transformers. Transformers 
is a very opinionated library, so there is a chance that
you don't agree with some of the library's philosophies or design
choices. From our experience, however, we found that the fundamental
design choices and philosophies of the library are crucial to
efficiently scale Transformers while keeping maintenance costs at a
reasonable level.

A good first starting point to better understand the library is to read
the [documentation of our philosophy](https://huggingface.co/transformers/philosophy.html).
As a result of our way of working, there are some choices that we try to apply to all models:

-   Composition is generally favored over abstraction
-   Duplicating code is not always bad if it strongly improves the
    readability or accessibility of a model
-   Model files are as self-contained as possible so that when you read
    the code of a specific model, you ideally only have to look into the
    respective `modeling_....py` file.

In our opinion, the library's code is not just a means to provide a
product, *e.g.*, the ability to use BERT for inference, but also as the
very product that we want to improve. Hence, when adding a model, the
user is not only the person that will use your model, but also everybody
that will read, try to understand, and possibly tweak your code.

With this in mind, let's go a bit deeper into the general library
design.

### Overview of models

To successfully add a model, it is important to understand the
interaction between your model and its config,
`PreTrainedModel`, and `PretrainedConfig`. For
exemplary purposes, we will call the PyTorch model to be added to 🤗 Transformers
`BrandNewBert`.

Let's take a look:

![image](../../../docs/source/imgs/transformers_overview.png)

As you can see, we do make use of inheritance in 🤗 Transformers, but we
keep the level of abstraction to an absolute minimum. There are never
more than two levels of abstraction for any model in the library.
`BrandNewBertModel` inherits from
`BrandNewBertPreTrainedModel` which in
turn inherits from `PreTrainedModel` and that's it. 
As a general rule, we want to make sure
that a new model only depends on `PreTrainedModel`. The
important functionalities that are automatically provided to every new
model are
`PreTrainedModel.from_pretrained` and `PreTrainedModel.save_pretrained`, which are 
used for serialization and deserialization. All
of the other important functionalities, such as
`BrandNewBertModel.forward` should be
completely defined in the new `modeling_brand_new_bert.py` module. Next,
we want to make sure that a model with a specific head layer, such as
`BrandNewBertForMaskedLM` does not inherit
from `BrandNewBertModel`, but rather uses
`BrandNewBertModel` as a component that
can be called in its forward pass to keep the level of abstraction low.
Every new model requires a configuration class, called
`BrandNewBertConfig`. This configuration
is always stored as an attribute in
`PreTrainedModel`, and
thus can be accessed via the `config` attribute for all classes
inheriting from `BrandNewBertPreTrainedModel`

```python
# assuming that `brand_new_bert` belongs to the organization `brandy`
model = BrandNewBertModel.from_pretrained("brandy/brand_new_bert")
model.config  # model has access to its config
```

Similar to the model, the configuration inherits basic serialization and
deserialization functionalities from
`PretrainedConfig`. Note
that the configuration and the model are always serialized into two
different formats - the model to a `pytorch_model.bin` file
and the configuration to a `config.json` file. Calling
`PreTrainedModel.save_pretrained` will automatically call
`PretrainedConfig.save_pretrained`, so that both model and configuration are saved.

### Overview of tokenizers

Not quite ready yet :-( This section will be added soon!

Step-by-step recipe to add a model to 🤗 Transformers
----------------------------------------------------

Everyone has different preferences of how to port a model so it can be
very helpful for you to take a look at summaries of how other
contributors ported models to Hugging Face. Here is a list of community
blog posts on how to port a model:

1.  [Porting GPT2
    Model](https://medium.com/huggingface/from-tensorflow-to-pytorch-265f40ef2a28)
    by [Thomas](https://huggingface.co/thomwolf)
2.  [Porting WMT19 MT Model](https://huggingface.co/blog/porting-fsmt)
    by [Stas](https://huggingface.co/stas)

From experience, we can tell you that the most important things to keep
in mind when adding a model are:

-   Don't reinvent the wheel! Most parts of the code you will add for
    the new 🤗 Transformers model already exist somewhere in 🤗
    Transformers. Take some time to find similar, already existing
    models and tokenizers you can copy from.
    [grep](https://www.gnu.org/software/grep/) and
    [rg](https://github.com/BurntSushi/ripgrep) are your friends. Note
    that it might very well happen that your model's tokenizer is based
    on one model implementation, and your model's modeling code on
    another one. *E.g.*, FSMT's modeling code is based on BART, while
    FSMT's tokenizer code is based on XLM.
-   It's more of an engineering challenge than a scientific challenge.
    You should spend more time on creating an efficient debugging
    environment than trying to understand all theoretical aspects of the
    model in the paper.
-   Ask for help when you're stuck! Models are the core component of 🤗
    Transformers so we, at Hugging Face, are more than happy to help
    you at every step to add your model. Don't hesitate to ask if you
    notice you are not making progress.

In the following, we try to give you a general recipe that we found most
useful when porting a model to 🤗 Transformers.

The following list is a summary of everything that has to be done to add
a model and can be used by you as a To-Do List:

1.  [ ] (Optional) Understood theoretical aspects

2.  [ ] Prepared transformers dev environment

3.  [ ] Set up debugging environment of the original repository

4.  [ ] Created script that successfully runs forward pass using
    original repository and checkpoint

5.  [ ] Successfully opened a PR and added the model skeleton to Transformers

6.  [ ] Successfully converted original checkpoint to Transformers
    checkpoint

7.  [ ] Successfully ran forward pass in Transformers that gives
    identical output to original checkpoint

8.  [ ] Finished model tests in Transformers

9.  [ ] Successfully added Tokenizer in Transformers

10. [ ] Run end-to-end integration tests

11. [ ] Finished docs

12. [ ] Uploaded model weights to the hub

13. [ ] Submitted the pull request for review

14. [ ] (Optional) Added a demo notebook

To begin with, we usually recommend to start by getting a good
theoretical understanding of `BigBird`. However, if you prefer to
understand the theoretical aspects of the model *on-the-job*, then it is
totally fine to directly dive into the `BigBird`'s code-base. This
option might suit you better, if your engineering skills are better than
your theoretical skill, if you have trouble understanding
`BigBird`'s paper, or if you just enjoy programming much more than
reading scientific papers.

### 1. (Optional) Theoretical aspects of BigBird

You should take some time to read *BigBird's* paper, if such
descriptive work exists. There might be large sections of the paper that
are difficult to understand. If this is the case, this is fine - don't
worry! The goal is not to get a deep theoretical understanding of the
paper, but to extract the necessary information required to effectively
re-implement the model in 🤗 Transformers. That being said, you don't
have to spend too much time on the theoretical aspects, but rather focus
on the practical ones, namely:

-   What type of model is *BigBird*? BERT-like encoder-only
    model? GPT2-like decoder-only model? BART-like encoder-decoder
    model? Look at the `model_summary` if
    you're not familiar with the differences between those.
-   What are the applications of *BigBird*? Text
    classification? Text generation? Seq2Seq tasks, *e.g.,*
    summarization?
-   What is the novel feature of the model making it different from
    BERT/GPT-2/BART?
-   Which of the already existing [🤗 Transformers
    models](https://huggingface.co/transformers/#contents) is most
    similar to *BigBird*?
-   What type of tokenizer is used? A sentencepiece tokenizer? Word
    piece tokenizer? Is it the same tokenizer as used for BERT or BART?

After you feel like you have gotten a good overview of the architecture
of the model, you might want to write to Patrick with any
questions you might have. This might include questions regarding the
model's architecture, its attention layer, etc. We will be more than
happy to help you.


#### Additional resources

 Before diving into the code, here are some additional resources that might be worth taking a look at:
 
 - [Yannic Kilcher's paper summary](https://www.youtube.com/watch?v=WVPE62Gk3EM&ab_channel=YannicKilcher)
 - [Yannic Kilcher's summary of Longformer](https://www.youtube.com/watch?v=_8KNb5iqblE&ab_channel=YannicKilcher) - Longformer and BigBird are **very** similar models. Since Longformer has already been ported to 🤗 Transformers, it is useful to understand the differences between the two models
 - [Blog post](https://medium.com/dsc-msit/is-google-bigbird-gonna-be-the-new-leader-in-nlp-domain-8c95cecc30f8) - A relatively superficial blog post about BigBird. Might be a good starting point to understand BigBird

#### Make sure you've understood the fundamental aspects of BigBird

Alright, now you should be ready to take a closer look into the actual code of BigBird.
You should have understood the following aspects of BigBird by now:

- BigBird provides a new attention layer for long-range sequence modelling that can be used 
  as a drop-in replacement for already existing architectures. This means that every transformer-based model architecture can replace its [Self-attention layer](https://towardsdatascience.com/illustrated-self-attention-2d627e33b20a) with BigBird's self-attention layer.
- BigBird's self-attention layer is composed of three mechanisms: block sparse (local) self-attention, global self-attention, random self-attention
- BigBird's block sparse (local) self-attention is different from Longformer's local self-attention. How so? Why does that matter? => Can be deployed on TPU much easier this way
- BigBird can be implemented for both an encoder-only model **and** 
  for an encoder-decoder model, which means that we can reuse lots of [code from RoBERTa](https://github.com/huggingface/transformers/blob/master/src/transformers/models/roberta/modeling_roberta.py) and [from PEGASUS](https://github.com/huggingface/transformers/blob/master/src/transformers/models/pegasus/modeling_pegasus.py) at a later stage.


If any of the mentioned aspects above are **not** clear to you, now is a great time to talk to Patrick.

### 2. Next prepare your environment

1.  Fork the [repository](https://github.com/huggingface/transformers)
    by clicking on the 'Fork' button on the repository's page. This
    creates a copy of the code under your GitHub user account.

2.  Clone your `transformers` fork to your local disk, and add the base
    repository as a remote:

    ```bash
    git clone https://github.com/[your Github handle]/transformers.git
    cd transformers
    git remote add upstream https://github.com/huggingface/transformers.git
    ```

3.  Set up a development environment, for instance by running the
    following command:

    ```bash
    python -m venv .env
    source .env/bin/activate
    pip install -e ".[dev]"
    ```

and return to the parent directory

```bash
cd ..
```

4.  We recommend adding the PyTorch version of *BigBird* to
    Transformers. To install PyTorch, please follow the instructions [here](https://pytorch.org/get-started/locally/).

**Note:** You don't need to have CUDA installed. Making the new model
work on CPU is sufficient.

5.  To port *BigBird*, you will also need access to its
    original repository:

```bash
git clone https://github.com/google-research/bigbird.git 
cd big_bird
pip install -e .
```

Now you have set up a development environment to port *BigBird*
to 🤗 Transformers.

### Run a pretrained checkpoint using the original repository

**3. Set up debugging environment**

At first, you will work on the original *BigBird* repository.
Often, the original implementation is very "researchy". Meaning that
documentation might be lacking and the code can be difficult to
understand. But this should be exactly your motivation to reimplement
*BigBird*. At Hugging Face, one of our main goals is to *make
people stand on the shoulders of giants* which translates here very well
into taking a working model and rewriting it to make it as **accessible,
user-friendly, and beautiful** as possible. This is the number-one
motivation to re-implement models into 🤗 Transformers - trying to make
complex new NLP technology accessible to **everybody**.

You should start thereby by diving into the [original repository](https://github.com/google-research/bigbird).

Successfully running the official pretrained model in the original
repository is often **the most difficult** step. From our experience, it
is very important to spend some time getting familiar with the original
code-base. You need to figure out the following:

-   Where to find the pretrained weights?
-   How to load the pretrained weights into the corresponding model?
-   How to run the tokenizer independently from the model?
-   Trace one forward pass so that you know which classes and functions
    are required for a simple forward pass. Usually, you only have to
    reimplement those functions.
-   Be able to locate the important components of the model: Where is
    the model's class? Are there model sub-classes, *e.g.*,
    EncoderModel, DecoderModel? Where is the self-attention layer? Are
    there multiple different attention layers, *e.g.*, *self-attention*,
    *cross-attention*...?
-   How can you debug the model in the original environment of the repo?
    Do you have to add `print` statements, can you work with
    an interactive debugger like [ipdb](https://pypi.org/project/ipdb/), or should you use
    an efficient IDE to debug the model, like PyCharm?

It is very important that before you start the porting process, that you
can **efficiently** debug code in the original repository! Also,
remember that you are working with an open-source library, so do not
hesitate to open an issue, or even a pull request in the original
repository. The maintainers of this repository are most likely very
happy about someone looking into their code!

At this point, it is really up to you which debugging environment and
strategy you prefer to use to debug the original model. We strongly
advise against setting up a costly GPU environment, but simply work on a
CPU both when starting to dive into the original repository and also
when starting to write the 🤗 Transformers implementation of the model.
Only at the very end, when the model has already been successfully
ported to 🤗 Transformers, one should verify that the model also works as
expected on GPU.

In general, there are two possible debugging environments for running
the original model

-   [Jupyter notebooks](https://jupyter.org/) / [google colab](https://colab.research.google.com/notebooks/intro.ipynb)
-   Local python scripts.

Jupyter notebooks have the advantage that they allow for cell-by-cell
execution which can be helpful to better split logical components from
one another and to have faster debugging cycles as intermediate results
can be stored. Also, notebooks are often easier to share with other
contributors, which might be very helpful if you want to ask the Hugging
Face team for help. If you are familiar with Jupiter notebooks, we
strongly recommend you to work with them.

The obvious disadvantage of Jupyther notebooks is that if you are not
used to working with them you will have to spend some time adjusting to
the new programming environment and that you might not be able to use
your known debugging tools anymore, like `ipdb`.

**4. Successfully run forward pass**

For each code-base, a good first step is always to load a **small**
pretrained checkpoint and to be able to reproduce a single forward pass
using a dummy integer vector of input IDs as an input. Such a script
could look something like this:

```python
from bigbird.core import modeling
model = modeling.BertModel(bert_config)
from bigbird.core import utils

params = utils.BigBirdConfig(vocab_size=32000, hidden_size=512,
    num_hidden_layers=8, num_attention_heads=6, intermediate_size=1024)

ckpt_path = 'gs://bigbird-transformer/pretrain/bigbr_base/model.ckpt-0'
ckpt_reader = tf.compat.v1.train.NewCheckpointReader(ckpt_path)
model.set_weights([ckpt_reader.get_tensor(v.name[:-2]) for v in tqdm(model.trainable_weights, position=0)])

input_ids = tf.constant([[31, 51, 99], [15, 5, 0]])
_, pooled_output = model(input_ids=input_ids, token_type_ids=token_type_ids)
...

```

Next, regarding the debugging strategy, there are generally a few from
which to choose from:

-   Decompose the original model into many small testable components and
    run a forward pass on each of those for verification
-   Decompose the original model only into the original *tokenizer* and
    the original *model*, run a forward pass on those, and use
    intermediate print statements or breakpoints for verification

Again, it is up to you which strategy to choose. Often, one or the other
is advantageous depending on the original code base.

If the original code-base allows you to decompose the model into smaller
sub-components, *e.g.*, if the original code-base can easily be run in
eager mode, it is usually worth the effort to do so. There are some
important advantages to taking the more difficult road in the beginning:

-   at a later stage when comparing the original model to the Hugging
    Face implementation, you can verify automatically for each component
    individually that the corresponding component of the 🤗 Transformers
    implementation matches instead of relying on visual comparison via
    print statements
-   it can give you some rope to decompose the big problem of porting a
    model into smaller problems of just porting individual components
    and thus structure your work better
-   separating the model into logical meaningful components will help
    you to get a better overview of the model's design and thus to
    better understand the model
-   at a later stage those component-by-component tests help you to
    ensure that no regression occurs as you continue changing your code

[Lysandre's](https://gist.github.com/LysandreJik/db4c948f6b4483960de5cbac598ad4ed)
integration checks for ELECTRA gives a nice example of how this can be
done.

However, if the original code-base is very complex or only allows
intermediate components to be run in a compiled mode, it might be too
time-consuming or even impossible to separate the model into smaller
testable sub-components. A good example is [T5's
MeshTensorFlow](https://github.com/tensorflow/mesh/tree/master/mesh_tensorflow)
library which is very complex and does not offer a simple way to
decompose the model into its sub-components. For such libraries, one
often relies on verifying print statements.

No matter which strategy you choose, the recommended procedure is often
the same in that you should start to debug the starting layers first and
the ending layers last.

It is recommended that you retrieve the output, either by print
statements or sub-component functions, of the following layers in the
following order:

1.  Retrieve the input IDs passed to the model
2.  Retrieve the word embeddings
3.  Retrieve the input of the first Transformer layer
4.  Retrieve the output of the first Transformer layer
5.  Retrieve the output of the following n - 1 Transformer layers
6.  Retrieve the output of the whole BigBird Model

Input IDs should thereby consists of an array of integers, *e.g.*,
`input_ids = [0, 4, 4, 3, 2, 4, 1, 7, 19]`

The outputs of the following layers often consist of multi-dimensional
float arrays and can look like this:

```bash
[[
 [-0.1465, -0.6501,  0.1993,  ...,  0.1451,  0.3430,  0.6024],
 [-0.4417, -0.5920,  0.3450,  ..., -0.3062,  0.6182,  0.7132],
 [-0.5009, -0.7122,  0.4548,  ..., -0.3662,  0.6091,  0.7648],
 ...,
 [-0.5613, -0.6332,  0.4324,  ..., -0.3792,  0.7372,  0.9288],
 [-0.5416, -0.6345,  0.4180,  ..., -0.3564,  0.6992,  0.9191],
 [-0.5334, -0.6403,  0.4271,  ..., -0.3339,  0.6533,  0.8694]]],
```

We expect that every model added to 🤗 Transformers passes a couple of
integration tests, meaning that the original model and the reimplemented
version in 🤗 Transformers have to give the exact same output up to a
precision of 0.001! Since it is normal that the exact same model written
in different libraries can give a slightly different output depending on
the library framework, we accept an error tolerance of 1e-3 (0.001). It
is not enough if the model gives nearly the same output, they have to be
the almost identical. Therefore, you will certainly compare the
intermediate outputs of the 🤗 Transformers version multiple times
against the intermediate outputs of the original implementation of
*BigBird* in which case an **efficient** debugging environment
of the original repository is absolutely important. Here is some advice
to make your debugging environment as efficient as possible.

-   Find the best way of debugging intermediate results. Is the original
    repository written in PyTorch? Then you should probably take the
    time to write a longer script that decomposes the original model
    into smaller sub-components to retrieve intermediate values. Is the
    original repository written in Tensorflow 1? Then you might have to
    rely on TensorFlow print operations like
    [tf.print](https://www.tensorflow.org/api_docs/python/tf/print) to
    output intermediate values. Is the original repository written in
    Jax? Then make sure that the model is **not jitted** when running
    the forward pass, *e.g.*, check-out [this
    link](https://github.com/google/jax/issues/196).
-   Use the smallest pretrained checkpoint you can find. The smaller the
    checkpoint, the faster your debug cycle becomes. It is not efficient
    if your pretrained model is so big that your forward pass takes more
    than 10 seconds. In case only very large checkpoints are available,
    it might make more sense to create a dummy model in the new
    environment with randomly initialized weights and save those weights
    for comparison with the 🤗 Transformers version of your model
-   Make sure you are using the easiest way of calling a forward pass in
    the original repository. Ideally, you want to find the function in
    the original repository that **only** calls a single forward pass,
    *i.e.* that is often called `predict`, `evaluate`, `forward` or
    `__call__`. You don't want to debug a function that calls `forward`
    multiple times, *e.g.*, to generate text, like
    `autoregressive_sample`, `generate`.
-   Try to separate the tokenization from the model's
    forward pass. If the original repository shows
    examples where you have to input a string, then try to find out
    where in the forward call the string input is changed to input ids
    and start from this point. This might mean that you have to possibly
    write a small script yourself or change the original code so that
    you can directly input the ids instead of an input string.
-   Make sure that the model in your debugging setup is **not** in
    training mode, which often causes the model to yield random outputs
    due to multiple dropout layers in the model. Make sure that the
    forward pass in your debugging environment is **deterministic** so
    that the dropout layers are not used. Or use
    `transformers.file_utils.set_seed` if the old and new
    implementations are in the same framework.

#### (Important) More details on how to create a debugging environment for BigBird 

- BigBird has multiple pretrained checkpoints that should eventually all be ported to 
  🤗 Transformers. The pretrained checkpoints can be found [here](https://console.cloud.google.com/storage/browser/bigbird-transformer/pretrain;tab=objects?prefix=&forceOnObjectsSortingFiltering=false). 
	Those checkpoints include both pretrained weights for encoder-only (BERT/RoBERTa) under the folder `bigbr_base` and encoder-decoder (PEGASUS) under the folder `bigbp_large`.
	You should start by porting the `bigbr_base` model. The encoder-decoder model 
	can be ported afterward.
	for an encoder-decoder architecture as well as an encoder-only architecture. 
- BigBird was written in tf.compat meaning that a mixture of a TensorFlow 1 and 
  TensorFlow 2 API was used.
- The most important part of the BigBird code-base is [bigbird.bigbird.core](https://github.com/google-research/bigbird/tree/master/bigbird/core) which includes all logic necessary 
  to implement BigBird.
- The first goal should be to successfully run a forward pass using the RoBERTa checkpoint `bigbr_base/model.ckpt-0.data-00000-of-00001` and `bigbr_base/model.ckpt-0.index`.


### Port BigBird to 🤗 Transformers

Next, you can finally start adding new code to 🤗 Transformers. Go into
the clone of your 🤗 Transformers' fork:

    cd transformers

In the special case that you are adding a model whose architecture
exactly matches the model architecture of an existing model you only
have to add a conversion script as described in [this
section](#write-a-conversion-script). In this case, you can just re-use
the whole model architecture of the already existing model.

Otherwise, let's start generating a new model with the amazing
Cookiecutter!

**Use the Cookiecutter to automatically generate the model's code**

To begin with head over to the [🤗 Transformers
templates](https://github.com/huggingface/transformers/tree/master/templates/adding_a_new_model)
to make use of our `cookiecutter` implementation to automatically
generate all the relevant files for your model. Again, we recommend only
adding the PyTorch version of the model at first. Make sure you follow
the instructions of the `README.md` on the [🤗 Transformers
templates](https://github.com/huggingface/transformers/tree/master/templates/adding_a_new_model)
carefully.
Since you will first implement the Encoder-only/RoBERTa-like version of BigBird you should 
select the `is_encoder_decoder_model = False` option in the cookiecutter. Also, it is recommended
that you implement the model only in PyTorch in the beginning and select "Standalone" as the 
tokenizer type for now.

**Open a Pull Request on the main huggingface/transformers repo**

Before starting to adapt the automatically generated code, now is the
time to open a "Work in progress (WIP)" pull request, *e.g.*, "\[WIP\]
Add *BigBird*", in 🤗 Transformers so that you and the Hugging
Face team can work side-by-side on integrating the model into 🤗
Transformers.

You should do the following:

1.  Create a branch with a descriptive name from your master branch

```
    git checkout -b add_big_bird
```

2.  Commit the automatically generated code:

```
    git add .
    git commit
```

3.  Fetch and rebase to current master

```
    git fetch upstream
    git rebase upstream/master
```

4.  Push the changes to your account using:

```
    git push -u origin a-descriptive-name-for-my-changes
```

5.  Once you are satisfied, go to the webpage of your fork on GitHub.
    Click on "Pull request". Make sure to add the GitHub handle of Patrick 
		as one reviewer, so that the Hugging Face team gets notified for future changes.

6.  Change the PR into a draft by clicking on "Convert to draft" on the
    right of the GitHub pull request web page.

In the following, whenever you have done some progress, don't forget to
commit your work and push it to your account so that it shows in the
pull request. Additionally, you should make sure to update your work
with the current master from time to time by doing:

    git fetch upstream
    git merge upstream/master

In general, all questions you might have regarding the model or your
implementation should be asked in your PR and discussed/solved in the
PR. This way, Patrick will always be notified when you are
committing new code or if you have a question. It is often very helpful
to point Patrick to your added code so that the Hugging
Face team can efficiently understand your problem or question.

To do so, you can go to the "Files changed" tab where you see all of
your changes, go to a line regarding which you want to ask a question,
and click on the "+" symbol to add a comment. Whenever a question or
problem has been solved, you can click on the "Resolve" button of the
created comment.

In the same way, Patrick will open comments when reviewing
your code. We recommend asking most questions on GitHub on your PR. For
some very general questions that are not very useful for the public,
feel free to ping Patrick by Slack or email.

**5. Adapt the generated models code for BigBird**

At first, we will focus only on the model itself and not care about the
tokenizer. All the relevant code should be found in the generated files
`src/transformers/models/big_bird/modeling_big_bird.py` and
`src/transformers/models/big_bird/configuration_big_bird.py`.

Now you can finally start coding :). The generated code in
`src/transformers/models/big_bird/modeling_big_bird.py` will
either have the same architecture as BERT if it's an encoder-only model
or BART if it's an encoder-decoder model. At this point, you should
remind yourself what you've learned in the beginning about the
theoretical aspects of the model: *How is the model different from BERT
or BART?*\". Implement those changes which often means to change the
*self-attention* layer, the order of the normalization layer, etc...
Again, it is often useful to look at the similar architecture of already
existing models in Transformers to get a better feeling of how your
model should be implemented.

**Note** that at this point, you don't have to be very sure that your
code is fully correct or clean. Rather, it is advised to add a first
*unclean*, copy-pasted version of the original code to
`src/transformers/models/big_bird/modeling_big_bird.py`
until you feel like all the necessary code is added. From our
experience, it is much more efficient to quickly add a first version of
the required code and improve/correct the code iteratively with the
conversion script as described in the next section. The only thing that
has to work at this point is that you can instantiate the 🤗 Transformers
implementation of *BigBird*, *i.e.* the following command
should work:

```python
from transformers import BigBirdModel, BigBirdConfig
model = BigBirdModel(BigBirdConfig())
```

The above command will create a model according to the default
parameters as defined in `BigBirdConfig()` with random weights,
thus making sure that the `init()` methods of all components works.

Note that for BigBird you have to change the attention layer. BigBird's attention
layer is quite complex as you can see [here](https://github.com/google-research/bigbird/blob/103a3345f94bf6364749b51189ed93024ca5ef26/bigbird/core/attention.py#L560). Don't 
feel discouraged by this! In a first step you should simply make sure that 
the layer `BigBirdAttention` has the correct weights as can be found in the 
pretrained checkpoints. This means that you have to make sure that in the 
`__init__(self, ...)` function of `BigBirdAttention`, all submodules include all 
necessary `nn.Module` layers. Only at a later stage do we need to fully rewrite 
the complex attention function.

**6. Write a conversion script**

Next, you should write a conversion script that lets you convert the
checkpoint you used to debug *BigBird* in the original
repository to a checkpoint compatible with your just created 🤗
Transformers implementation of *BigBird*. It is not advised to
write the conversion script from scratch, but rather to look through
already existing conversion scripts in 🤗 Transformers for one that has
been used to convert a similar model that was written in the same
framework as *BigBird*. Usually, it is enough to copy an
already existing conversion script and slightly adapt it for your use
case. Don't hesitate to ask Patrick to point you to a
similar already existing conversion script for your model.

-   A good starting point to convert the original TF BigBird implementation to the PT Hugging Face implementation is probably BERT's conversion script
    [here](https://github.com/huggingface/transformers/blob/7acfa95afb8194f8f9c1f4d2c6028224dbed35a2/src/transformers/models/bert/modeling_bert.py#L91)

You can copy paste the conversion function into `modeling_big_bird.py` and then adapt it 
to your needs.

In the following, we'll quickly explain how PyTorch models store layer
weights and define layer names. In PyTorch, the name of a layer is
defined by the name of the class attribute you give the layer. Let's
define a dummy model in PyTorch, called `SimpleModel` as follows:

```python
import torch.nn as nn

class SimpleModel(nn.Module):
    def __init__(self):
            super().__init__()
            self.dense = nn.Linear(10, 10)
            self.intermediate = nn.Linear(10, 10)
            self.layer_norm = nn.LayerNorm(10)
```

Now we can create an instance of this model definition which will fill
all weights: `dense`, `intermediate`, `layer_norm` with random weights.
We can print the model to see its architecture

```python
model = SimpleModel()

print(model)
```

This will print out the following:

```bash
SimpleModel(
  (dense): Linear(in_features=10, out_features=10, bias=True)
  (intermediate): Linear(in_features=10, out_features=10, bias=True)
  (layer_norm): LayerNorm((10,), eps=1e-05, elementwise_affine=True)
)
```

We can see that the layer names are defined by the name of the class
attribute in PyTorch. You can print out the weight values of a specific
layer:

```python
print(model.dense.weight.data)
```

to see that the weights were randomly initialized

```bash
tensor([[-0.0818,  0.2207, -0.0749, -0.0030,  0.0045, -0.1569, -0.1598,  0.0212,
         -0.2077,  0.2157],
        [ 0.1044,  0.0201,  0.0990,  0.2482,  0.3116,  0.2509,  0.2866, -0.2190,
          0.2166, -0.0212],
        [-0.2000,  0.1107, -0.1999, -0.3119,  0.1559,  0.0993,  0.1776, -0.1950,
         -0.1023, -0.0447],
        [-0.0888, -0.1092,  0.2281,  0.0336,  0.1817, -0.0115,  0.2096,  0.1415,
         -0.1876, -0.2467],
        [ 0.2208, -0.2352, -0.1426, -0.2636, -0.2889, -0.2061, -0.2849, -0.0465,
          0.2577,  0.0402],
        [ 0.1502,  0.2465,  0.2566,  0.0693,  0.2352, -0.0530,  0.1859, -0.0604,
          0.2132,  0.1680],
        [ 0.1733, -0.2407, -0.1721,  0.1484,  0.0358, -0.0633, -0.0721, -0.0090,
          0.2707, -0.2509],
        [-0.1173,  0.1561,  0.2945,  0.0595, -0.1996,  0.2988, -0.0802,  0.0407,
          0.1829, -0.1568],
        [-0.1164, -0.2228, -0.0403,  0.0428,  0.1339,  0.0047,  0.1967,  0.2923,
          0.0333, -0.0536],
        [-0.1492, -0.1616,  0.1057,  0.1950, -0.2807, -0.2710, -0.1586,  0.0739,
          0.2220,  0.2358]]).
```

In the conversion script, you should fill those randomly initialized
weights with the exact weights of the corresponding layer in the
checkpoint. *E.g.*,

```python
# retrieve matching layer weights, e.g. by 
# recursive algorithm
layer_name = "dense"
pretrained_weight = array_of_dense_layer

model_pointer = getattr(model, "dense")

model_pointer.weight.data = torch.from_numpy(pretrained_weight)
```

While doing so, you must verify that each randomly initialized weight of
your PyTorch model and its corresponding pretrained checkpoint weight
exactly match in both **shape and name**. To do so, it is **necessary**
to add assert statements for the shape and print out the names of the
checkpoints weights. *E.g.*, you should add statements like:

```python
assert (
     model_pointer.weight.shape == pretrained_weight.shape
), f"Pointer shape of random weight {model_pointer.shape} and array shape of checkpoint weight {pretrained_weight.shape} mismatched"
```

Besides, you should also print out the names of both weights to make
sure they match, *e.g.*,

```python
logger.info(f"Initialize PyTorch weight {layer_name} from {pretrained_weight.name}")
```

If either the shape or the name doesn't match, you probably assigned
the wrong checkpoint weight to a randomly initialized layer of the 🤗
Transformers implementation.

An incorrect shape is most likely due to an incorrect setting of the
config parameters in `BigBirdConfig()` that do not exactly match
those that were used for the checkpoint you want to convert. However, it
could also be that PyTorch's implementation of a layer requires the
weight to be transposed beforehand.

Finally, you should also check that **all** required weights are
initialized and print out all checkpoint weights that were not used for
initialization to make sure the model is correctly converted. It is
completely normal, that the conversion trials fail with either a wrong
shape statement or wrong name assignment. This is most likely because
either you used incorrect parameters in `BigBirdConfig()`, have a
wrong architecture in the 🤗 Transformers implementation, you have a bug
in the `init()` functions of one of the components of the 🤗 Transformers
implementation or you need to transpose one of the checkpoint weights.

This step should be iterated with the previous step until all weights of
the checkpoint are correctly loaded in the Transformers model. Having
correctly loaded the checkpoint into the 🤗 Transformers implementation,
you can then save the model under a folder of your choice
`/path/to/converted/checkpoint/folder` that should then contain both a
`pytorch_model.bin` file and a `config.json` file:

```python
model.save_pretrained("/path/to/converted/checkpoint/folder")
```

**7. Implement the forward pass**

Having managed to correctly load the pretrained weights into the 🤗
Transformers implementation, you should now make sure that the forward
pass is correctly implemented. In [Get familiar with the original
repository](#run-a-pretrained-checkpoint-using-the-original-repository),
you have already created a script that runs a forward pass of the model
using the original repository. Now you should write an analogous script
using the 🤗 Transformers implementation instead of the original one. It
should look as follows:

[Here the model name might have to be adapted, *e.g.*, maybe BigBirdForConditionalGeneration instead of BigBirdModel]

```python
model = BigBirdModel.from_pretrained("/path/to/converted/checkpoint/folder")
input_ids = [0, 4, 4, 3, 2, 4, 1, 7, 19]
output = model(input_ids).last_hidden_states
```

It is very likely that the 🤗 Transformers implementation and the
original model implementation don't give the exact same output the very
first time or that the forward pass throws an error. Don't be
disappointed - it's expected! First, you should make sure that the
forward pass doesn't throw any errors. It often happens that the wrong
dimensions are used leading to a `"Dimensionality mismatch"`
error or that the wrong data type object is used, *e.g.*, `torch.long`
instead of `torch.float32`. Don't hesitate to ask Patrick
for help, if you don't manage to solve certain errors.

The final part to make sure the 🤗 Transformers implementation works
correctly is to ensure that the outputs are equivalent to a precision of
`1e-3`. First, you should ensure that the output shapes are identical,
*i.e.* `outputs.shape` should yield the same value for the script of the
🤗 Transformers implementation and the original implementation. Next, you
should make sure that the output values are identical as well. This one
of the most difficult parts of adding a new model. Common mistakes why
the outputs are not identical are:

-   Some layers were not added, *i.e.* an activation layer
    was not added, or the residual connection was forgotten
-   The word embedding matrix was not tied
-   The wrong positional embeddings are used because the original
    implementation uses on offset
-   Dropout is applied during the forward pass. To fix this make sure
    `model.training is False` and that no dropout layer is
    falsely activated during the forward pass, *i.e.* pass
    `self.training` to [PyTorch's functional
    dropout](https://pytorch.org/docs/stable/nn.functional.html?highlight=dropout#torch.nn.functional.dropout)

The best way to fix the problem is usually to look at the forward pass
of the original implementation and the 🤗 Transformers implementation
side-by-side and check if there are any differences. Ideally, you should
debug/print out intermediate outputs of both implementations of the
forward pass to find the exact position in the network where the 🤗
Transformers implementation shows a different output than the original
implementation. First, make sure that the hard-coded `input_ids` in both
scripts are identical. Next, verify that the outputs of the first
transformation of the `input_ids` (usually the word embeddings) are
identical. And then work your way up to the very last layer of the
network. At some point, you will notice a difference between the two
implementations, which should point you to the bug in the 🤗 Transformers
implementation. From our experience, a simple and efficient way is to
add many print statements in both the original implementation and 🤗
Transformers implementation, at the same positions in the network
respectively, and to successively remove print statements showing the
same values for intermediate presentions.

When you're confident that both implementations yield the same output,
verifying the outputs with
`torch.allclose(original_output, output, atol=1e-3)`, you're done with
the most difficult part! Congratulations - the work left to be done
should be a cakewalk 😊.

**8. Adding all necessary model tests**

At this point, you have successfully added a new model. However, it is
very much possible that the model does not yet fully comply with the
required design. To make sure, the implementation is fully compatible
with 🤗 Transformers, all common tests should pass. The Cookiecutter
should have automatically added a test file for your model, probably
under the same `tests/test_modeling_big_bird.py`. Run this test
file to verify that all common tests pass:

```python
pytest tests/test_modeling_big_bird.py
```

Having fixed all common tests, it is now crucial to ensure that all the
nice work you have done is well tested, so that

-   a)  The community can easily understand your work by looking at
        specific tests of *BigBird*

-   b)  Future changes to your model will not break any important
        feature of the model.

At first, integration tests should be added. Those integration tests
essentially do the same as the debugging scripts you used earlier to
implement the model to 🤗 Transformers. A template of those model tests
is already added by the Cookiecutter, called
`BigBirdModelIntegrationTests` and only has to be filled out by
you. To ensure that those tests are passing, run

```python
RUN_SLOW=1 pytest -sv tests/test_modeling_big_bird.py::BigBirdModelIntegrationTests
```

**Note**: In case you are using Windows, you should replace `RUN_SLOW=1` with
`SET RUN_SLOW=1`

Second, all features that are special to *BigBird* should be
tested additionally in a separate test under
`BigBirdModelTester`/`BigBirdModelTest`. This part is often
forgotten but is extremely useful in two ways:

-   It helps to transfer the knowledge you have acquired during the
    model addition to the community by showing how the special features
    of *BigBird* should work.
-   Future contributors can quickly test changes to the model by running
    those special tests.

BigBird has quite a complex attention layer, so it is very important 
to add more tests verifying the all parts of BigBird's self-attention layer 
works as expected. This means that there should be at least 3 additional tests:

- 1. Verify that the sparse attention works correctly
- 2. Verify that the global attention works correctly
- 3. Verify that the random attention works correctly

**9. Implement the tokenizer**

Next, we should add the tokenizer of *BigBird*. Usually, the
tokenizer is equivalent or very similar to an already existing tokenizer
of 🤗 Transformers.

In the case of BigBird you should be able to just rely on an already existing tokenizer.
If not mistaken, BigBird uses the same tokenizer that was used for `BertGenerationTokenizer`,
which is based on `sentencepiece`. So you should be able to just set the config parameter 
`tokenizer_class` to `BertGenerationTokenizer` without having to implement any new tokenizer.

It is very important to find/extract the original tokenizer file and to
manage to load this file into the 🤗 Transformers' implementation of the
tokenizer.

For BigBird, the tokenizer (sentencepiece) files can be found [here](https://github.com/google-research/bigbird/blob/master/bigbird/vocab/gpt2.model), which you should be able to load 
as easily as:

```python
from transformers import BertGenerationTokenizer
tokenizer = BertGenerationTokenizer("/path/to/gpt2.model/file")
```

To ensure that the tokenizer works correctly, it is recommended to first
create a script in the original repository that inputs a string and
returns the `input_ids`. It could look similar to this (in pseudo-code):

```bash
input_str = "This is a long example input string containing special characters .$?-, numbers 2872 234 12 and words."
model = BigBirdModel.load_pretrained_checkpoint("/path/to/checkpoint/")
input_ids = model.tokenize(input_str)
```

You might have to take a deeper look again into the original repository
to find the correct tokenizer function or you might even have to do
changes to your clone of the original repository to only output the
`input_ids`. Having written a functional tokenization script that uses
the original repository, an analogous script for 🤗 Transformers should
be created. It should look similar to this:

```python
from transformers import BertGenerationTokenizer
input_str = "This is a long example input string containing special characters .$?-, numbers 2872 234 12 and words."

tokenizer = BertGenerationTokenizer.from_pretrained("/path/big/bird/folder")

input_ids = tokenizer(input_str).input_ids
```

When both `input_ids` yield the same values, as a final step a tokenizer
test file should also be added.

Since BigBird is most likely fully based on `BertGenerationTokenizer`, 
you should only add a couple of "slow" integration tests. However, in this 
case you do **not** need to add any `BigBirdTokenizationTest`.

**10. Run End-to-end integration tests**

Having added the tokenizer, you should also add a couple of end-to-end
integration tests using both the model and the tokenizer to
`tests/test_modeling_big_bird.py` in 🤗 Transformers. Such a test
should show on a meaningful text-to-text sample that the 🤗 Transformers
implementation works as expected. A meaningful text-to-text sample can
include, *e.g.*, a source-to-target-translation pair, an
article-to-summary pair, a question-to-answer pair, etc... If none of
the ported checkpoints has been fine-tuned on a downstream task it is
enough to simply rely on the model tests. In a final step to ensure that
the model is fully functional, it is advised that you also run all tests
on GPU. It can happen that you forgot to add some `.to(self.device)`
statements to internal tensors of the model, which in such a test would
show in an error. In case you have no access to a GPU, the Hugging Face
team can take care of running those tests for you.

**11. Add Docstring**

Now, all the necessary functionality for *BigBird* is added -
you're almost done! The only thing left to add is a nice docstring and
a doc page. The Cookiecutter should have added a template file called
`docs/source/model_doc/big_bird.rst` that you should fill out.
Users of your model will usually first look at this page before using
your model. Hence, the documentation must be understandable and concise.
It is very useful for the community to add some *Tips* to show how the
model should be used. Don't hesitate to ping Patrick
regarding the docstrings.

Next, make sure that the docstring added to
`src/transformers/models/big_bird/modeling_big_bird.py` is
correct and included all necessary inputs and outputs. It is always to
good to remind oneself that documentation should be treated at least as
carefully as the code in 🤗 Transformers since the documentation is
usually the first contact point of the community with the model.

**Code refactor**

Great, now you have added all the necessary code for *BigBird*.
At this point, you should correct some potential incorrect code style by
running:

```bash
make style
```

and verify that your coding style passes the quality check:

```bash
make quality
```

There are a couple of other very strict design tests in 🤗 Transformers
that might still be failing, which shows up in the tests of your pull
request. This is often because of some missing information in the
docstring or some incorrect naming. Patrick will surely
help you if you're stuck here.

Lastly, it is always a good idea to refactor one's code after having
ensured that the code works correctly. With all tests passing, now it's
a good time to go over the added code again and do some refactoring.

You have now finished the coding part, congratulation! 🎉 You are
Awesome! 😎

**12. Upload the models to the model hub**

In this final part, you should convert and upload all checkpoints to the
model hub and add a model card for each uploaded model checkpoint. You
should work alongside Patrick here to decide on a fitting
name for each checkpoint and to get the required access rights to be
able to upload the model under the author's organization of
*BigBird*.

It is worth spending some time to create fitting model cards for each
checkpoint. The model cards should highlight the specific
characteristics of this particular checkpoint, *e.g.*, On which dataset
was the checkpoint pretrained/fine-tuned on? On what down-stream task
should the model be used? And also include some code on how to correctly
use the model.

**13. (Optional) Add notebook**

It is very helpful to add a notebook that showcases in-detail how
*BigBird* can be used for inference and/or fine-tuned on a
downstream task. This is not mandatory to merge your PR, but very useful
for the community.

**14. Submit your finished PR**

You're done programming now and can move to the last step, which is
getting your PR merged into master. Usually, Patrick
should have helped you already at this point, but it is worth taking
some time to give your finished PR a nice description and eventually add
comments to your code, if you want to point out certain design choices
to your reviewer.

### Share your work!!

Now, it's time to get some credit from the community for your work!
Having completed a model addition is a major contribution to
Transformers and the whole NLP community. Your code and the ported
pre-trained models will certainly be used by hundreds and possibly even
thousands of developers and researchers. You should be proud of your
work and share your achievement with the community.

**You have made another model that is super easy to access for everyone
in the community! 🤯**
