add instructions how to add tests for prototype datasets (#5666)

torchvision/prototype/datasets/_builtin/README.md

 # How to add new built-in prototype datasets
 
-As the name implies, the datasets are still in a prototype state and thus
-subject to rapid change. This in turn means that this document will also change
-a lot.
+As the name implies, the datasets are still in a prototype state and thus subject to rapid change. This in turn means
+that this document will also change a lot.
 
-If you hit a blocker while adding a dataset, please have a look at another
-similar dataset to see how it is implemented there. If you can't resolve it
-yourself, feel free to send a draft PR in order for us to help you out.
+If you hit a blocker while adding a dataset, please have a look at another similar dataset to see how it is implemented
+there. If you can't resolve it yourself, feel free to send a draft PR in order for us to help you out.
 
 Finally, `from torchvision.prototype import datasets` is implied below.
 
 ## Implementation
 
-Before we start with the actual implementation, you should create a module in
-`torchvision/prototype/datasets/_builtin` that hints at the dataset you are
-going to add. For example `caltech.py` for `caltech101` and `caltech256`. In
-that module create a class that inherits from `datasets.utils.Dataset` and
-overwrites at minimum three methods that will be discussed in detail below:
+Before we start with the actual implementation, you should create a module in `torchvision/prototype/datasets/_builtin`
+that hints at the dataset you are going to add. For example `caltech.py` for `caltech101` and `caltech256`. In that
+module create a class that inherits from `datasets.utils.Dataset` and overwrites at minimum three methods that will be
+discussed in detail below:
 
 ```python
 from typing import Any, Dict, List
@@ -39,50 +36,39 @@ class MyDataset(Dataset):
 
 ### `_make_info(self)`
 
-The `DatasetInfo` carries static information about the dataset. There are two
-required fields:
-- `name`: Name of the dataset. This will be used to load the dataset with
-  `datasets.load(name)`. Should only contain lowercase characters.
+The `DatasetInfo` carries static information about the dataset. There are two required fields:
+
+- `name`: Name of the dataset. This will be used to load the dataset with `datasets.load(name)`. Should only contain
+  lowercase characters.
 
 There are more optional parameters that can be passed:
 
-- `dependencies`: Collection of third-party dependencies that are needed to load
-  the dataset, e.g. `("scipy",)`. Their availability will be automatically
-  checked if a user tries to load the dataset. Within the implementation, import
+- `dependencies`: Collection of third-party dependencies that are needed to load the dataset, e.g. `("scipy",)`. Their
+  availability will be automatically checked if a user tries to load the dataset. Within the implementation, import
   these packages lazily to avoid missing dependencies at import time.
-- `categories`: Sequence of human-readable category names for each label. The
-  index of each category has to match the corresponding label returned in the
-  dataset samples. [See
-  below](#how-do-i-handle-a-dataset-that-defines-many-categories) how to handle
-  cases with many categories.
-- `valid_options`: Configures valid options that can be passed to the dataset.
-  It should be `Dict[str, Sequence[Any]]`. The options are accessible through
-  the `config` namespace in the other two functions. First value of the sequence
-  is taken as default if the user passes no option to
-  `torchvision.prototype.datasets.load()`.
+- `categories`: Sequence of human-readable category names for each label. The index of each category has to match the
+  corresponding label returned in the dataset samples.
+  [See below](#how-do-i-handle-a-dataset-that-defines-many-categories) how to handle cases with many categories.
+- `valid_options`: Configures valid options that can be passed to the dataset. It should be `Dict[str, Sequence[Any]]`.
+  The options are accessible through the `config` namespace in the other two functions. First value of the sequence is
+  taken as default if the user passes no option to `torchvision.prototype.datasets.load()`.
 
 ## `resources(self, config)`
 
-Returns `List[datasets.utils.OnlineResource]` of all the files that need to be
-present locally before the dataset with a specific `config` can be build. The
-download will happen automatically. 
+Returns `List[datasets.utils.OnlineResource]` of all the files that need to be present locally before the dataset with a
+specific `config` can be build. The download will happen automatically.
 
 Currently, the following `OnlineResource`'s are supported:
 
-- `HttpResource`: Used for files that are directly exposed through HTTP(s) and
-  only requires the URL.
-- `GDriveResource`: Used for files that are hosted on GDrive and requires the
-  GDrive ID as well as the `file_name`.
-- `ManualDownloadResource`: Used files are not publicly accessible and requires
-  instructions how to download them manually. If the file does not exist, an
-  error will be raised with the supplied instructions.
-- `KaggleDownloadResource`: Used for files that are available on Kaggle. This
-  inherits from `ManualDownloadResource`.
-
-Although optional in general, all resources used in the built-in datasets should
-comprise [SHA256](https://en.wikipedia.org/wiki/SHA-2) checksum for security. It
-will be automatically checked after the download. You can compute the checksum
-with system utilities e.g `sha256-sum`, or this snippet:
+- `HttpResource`: Used for files that are directly exposed through HTTP(s) and only requires the URL.
+- `GDriveResource`: Used for files that are hosted on GDrive and requires the GDrive ID as well as the `file_name`.
+- `ManualDownloadResource`: Used files are not publicly accessible and requires instructions how to download them
+  manually. If the file does not exist, an error will be raised with the supplied instructions.
+- `KaggleDownloadResource`: Used for files that are available on Kaggle. This inherits from `ManualDownloadResource`.
+
+Although optional in general, all resources used in the built-in datasets should comprise
+[SHA256](https://en.wikipedia.org/wiki/SHA-2) checksum for security. It will be automatically checked after the
+download. You can compute the checksum with system utilities e.g `sha256-sum`, or this snippet:
 
 ```python
 import hashlib
@@ -97,61 +83,123 @@ def sha256sum(path, chunk_size=1024 * 1024):
 
 ### `_make_datapipe(resource_dps, *, config)`
 
-This method is the heart of the dataset, where we transform the raw data into
-a usable form. A major difference compared to the current stable datasets is
-that everything is performed through `IterDataPipe`'s. From the perspective of
-someone that is working with them rather than on them, `IterDataPipe`'s behave
-just as generators, i.e. you can't do anything with them besides iterating.
+This method is the heart of the dataset, where we transform the raw data into a usable form. A major difference compared
+to the current stable datasets is that everything is performed through `IterDataPipe`'s. From the perspective of someone
+that is working with them rather than on them, `IterDataPipe`'s behave just as generators, i.e. you can't do anything
+with them besides iterating.
 
-Of course, there are some common building blocks that should suffice in 95% of
-the cases. The most used are:
+Of course, there are some common building blocks that should suffice in 95% of the cases. The most used are:
 
 - `Mapper`: Apply a callable to every item in the datapipe.
 - `Filter`: Keep only items that satisfy a condition.
 - `Demultiplexer`: Split a datapipe into multiple ones.
 - `IterKeyZipper`: Merge two datapipes into one.
 
-All of them can be imported `from torchdata.datapipes.iter`. In addition, use
-`functools.partial` in case a callable needs extra arguments.  If the provided
-`IterDataPipe`'s are not sufficient for the use case, it is also not complicated
+All of them can be imported `from torchdata.datapipes.iter`. In addition, use `functools.partial` in case a callable
+needs extra arguments. If the provided `IterDataPipe`'s are not sufficient for the use case, it is also not complicated
 to add one. See the MNIST or CelebA datasets for example.
 
-`make_datapipe()` receives `resource_dps`, which is a list of datapipes that has
-a 1-to-1 correspondence with the return value of `resources()`. In case of
-archives with regular suffixes (`.tar`, `.zip`, ...), the datapipe will contain
-tuples comprised of the path and the handle for every file in the archive.
-Otherwise the datapipe will only contain one of such tuples for the file
-specified by the resource.
+`make_datapipe()` receives `resource_dps`, which is a list of datapipes that has a 1-to-1 correspondence with the return
+value of `resources()`. In case of archives with regular suffixes (`.tar`, `.zip`, ...), the datapipe will contain
+tuples comprised of the path and the handle for every file in the archive. Otherwise the datapipe will only contain one
+of such tuples for the file specified by the resource.
+
+Since the datapipes are iterable in nature, some datapipes feature an in-memory buffer, e.g. `IterKeyZipper` and
+`Grouper`. There are two issues with that: 1. If not used carefully, this can easily overflow the host memory, since
+most datasets will not fit in completely. 2. This can lead to unnecessarily long warm-up times when data is buffered
+that is only needed at runtime.
+
+Thus, all buffered datapipes should be used as early as possible, e.g. zipping two datapipes of file handles rather than
+trying to zip already loaded images.
+
+There are two special datapipes that are not used through their class, but through the functions `hint_sharding` and
+`hint_shuffling`. As the name implies they only hint part in the datapipe graph where sharding and shuffling should take
+place, but are no-ops by default. They can be imported from `torchvision.prototype.datasets.utils._internal` and are
+required in each dataset.
+
+Finally, each item in the final datapipe should be a dictionary with `str` keys. There is no standardization of the
+names (yet!).
+
+## Tests
+
+To test the dataset implementation, you usually don't need to add any tests, but need to provide a mock-up of the data.
+This mock-up should resemble the original data as close as necessary, while containing only few examples.
+
+To do this, add a new function in [`test/builtin_dataset_mocks.py`](../../../../test/builtin_dataset_mocks.py) with the
+same name as you have defined in `_make_config()` (if the name includes hyphens `-`, replace them with underscores `_`)
+and decorate it with `@register_mock`:
+
+```py
+# this is defined in torchvision/prototype/datasets/_builtin
+class MyDataset(Dataset):
+    def _make_info(self) -> DatasetInfo:
+        return DatasetInfo(
+          "my-dataset",
+          ...
+        )
+
+@register_mock
+def my_dataset(info, root, config):
+    ...
+```
 
-Since the datapipes are iterable in nature, some datapipes feature an in-memory
-buffer, e.g. `IterKeyZipper` and `Grouper`. There are two issues with that: 1.
-If not used carefully, this can easily overflow the host memory, since most
-datasets will not fit in completely. 2. This can lead to unnecessarily long
-warm-up times when data is buffered that is only needed at runtime.
+The function receives three arguments:
 
-Thus, all buffered datapipes should be used as early as possible, e.g. zipping
-two datapipes of file handles rather than trying to zip already loaded images.
+- `info`: The return value of `_make_info()`.
+- `root`: A [`pathlib.Path`](https://docs.python.org/3/library/pathlib.html#pathlib.Path) of a folder, in which the data
+  needs to be placed.
+- `config`: The configuration to generate the data for. This is the same value that `_make_datapipe()` receives.
 
-There are two special datapipes that are not used through their class, but
-through the functions `hint_sharding` and `hint_shuffling`. As the name implies
-they only hint part in the datapipe graph where sharding and shuffling should
-take place, but are no-ops by default. They can be imported from
-`torchvision.prototype.datasets.utils._internal` and are required in each
-dataset.
+The function should generate all files that are needed for the current `config`. Each file should be complete, e.g. if
+the dataset only has a single archive that contains multiple splits, you need to generate all regardless of the current
+`config`. Although this seems odd at first, this is important. Consider the following original data setup:
 
-Finally, each item in the final datapipe should be a dictionary with `str` keys.
-There is no standardization of the names (yet!).
+```
+root
+├── test
+│   ├── test_image0.jpg
+│   ...
+└── train
+    ├── train_image0.jpg
+    ...
+```
+
+For map-style datasets (like the one currently in `torchvision.datasets`), one explicitly selects the files they want to
+load. For example, something like `(root / split).iterdir()` works fine even if only the specific split folder is
+present. With iterable-style datasets though, we get something like `root.iterdir()` from `resource_dps` in
+`_make_datapipe()` and need to manually `Filter` it to only keep the files we want. If we would only generate the data
+for the current `config`, the test would also pass if the dataset is missing the filtering, but would fail on the real
+data.
+
+For datasets that are ported from the old API, we already have some mock data in
+[`test/test_datasets.py`](../../../../test/test_datasets.py). You can find the test case corresponding test case there
+and have a look at the `inject_fake_data` function. There are a few differences though:
+
+- `tmp_dir` corresponds to `root`, but is a `str` rather than a
+  [`pathlib.Path`](https://docs.python.org/3/library/pathlib.html#pathlib.Path). Thus, you often see something like
+  `folder = pathlib.Path(tmp_dir)`. This is not needed.
+- Although both parameters are called `config`, the value in the new tests is a namespace. Thus, please use `config.foo`
+  over `config["foo"]` to enhance readability.
+- The data generated by `inject_fake_data` was supposed to be in an extracted state. This is no longer the case for the
+  new mock-ups. Thus, you need to use helper functions like `make_zip` or `make_tar` to actually generate the files
+  specified in the dataset.
+- As explained in the paragraph above, the generated data is often "incomplete" and only valid for given the config.
+  Make sure you follow the instructions above.
+
+The function should return an integer indicating the number of samples in the dataset for the current `config`.
+Preferably, this number should be different for different `config`'s to have more confidence in the dataset
+implementation.
+
+Finally, you can run the tests with `pytest test/test_prototype_builtin_datasets.py -k {name}`.
 
 ## FAQ
 
 ### How do I start?
 
-Get the skeleton of your dataset class ready with all 3 methods. For
-`_make_datapipe()`, you can just do `return resources_dp[0]` to get started.
-Then import the dataset class in
-`torchvision/prototype/datasets/_builtin/__init__.py`: this will automatically
-register the dataset and it will be instantiable via
-`datasets.load("mydataset")`. On a separate script, try something like
+Get the skeleton of your dataset class ready with all 3 methods. For `_make_datapipe()`, you can just do
+`return resources_dp[0]` to get started. Then import the dataset class in
+`torchvision/prototype/datasets/_builtin/__init__.py`: this will automatically register the dataset and it will be
+instantiable via `datasets.load("mydataset")`. On a separate script, try something like
 
 ```py
 from torchvision.prototype import datasets
@@ -163,35 +211,27 @@ for sample in dataset:
 # Or you can also inspect the sample in a debugger
 ```
 
-This will give you an idea of what the first datapipe in `resources_dp`
-contains. You can also do that with `resources_dp[1]` or `resources_dp[2]`
-(etc.) if they exist. Then follow the instructions above to manipulate these
+This will give you an idea of what the first datapipe in `resources_dp` contains. You can also do that with
+`resources_dp[1]` or `resources_dp[2]` (etc.) if they exist. Then follow the instructions above to manipulate these
 datapipes and return the appropriate dictionary format.
 
 ### How do I handle a dataset that defines many categories?
 
-As a rule of thumb, `datasets.utils.DatasetInfo(..., categories=)` should only
-be set directly for ten categories or fewer. If more categories are needed, you
-can add a `$NAME.categories` file to the `_builtin` folder in which each line
-specifies a category. If `$NAME` matches the name of the dataset (which it
-definitively should!) it will be automatically loaded if `categories=` is not
-set.
-
-In case the categories can be generated from the dataset files, e.g. the dataset
-follows an image folder approach where each folder denotes the name of the
-category, the dataset can overwrite the `_generate_categories` method. It gets
-passed the `root` path to the resources, but they have to be manually loaded,
-e.g. `self.resources(config)[0].load(root)`. The method should return a sequence
-of strings representing the category names. To generate the `$NAME.categories`
-file, run `python -m torchvision.prototype.datasets.generate_category_files
-$NAME`.
+As a rule of thumb, `datasets.utils.DatasetInfo(..., categories=)` should only be set directly for ten categories or
+fewer. If more categories are needed, you can add a `$NAME.categories` file to the `_builtin` folder in which each line
+specifies a category. If `$NAME` matches the name of the dataset (which it definitively should!) it will be
+automatically loaded if `categories=` is not set.
+
+In case the categories can be generated from the dataset files, e.g. the dataset follows an image folder approach where
+each folder denotes the name of the category, the dataset can overwrite the `_generate_categories` method. It gets
+passed the `root` path to the resources, but they have to be manually loaded, e.g.
+`self.resources(config)[0].load(root)`. The method should return a sequence of strings representing the category names.
+To generate the `$NAME.categories` file, run `python -m torchvision.prototype.datasets.generate_category_files $NAME`.
 
 ### What if a resource file forms an I/O bottleneck?
 
-In general, we are ok with small performance hits of iterating archives rather
-than their extracted content. However, if the performance hit becomes
-significant, the archives can still be decompressed or extracted. To do this,
-the `decompress: bool` and `extract: bool` flags can be used for every
-`OnlineResource` individually. For more complex cases, each resource also
-accepts a `preprocess` callable that gets passed a `pathlib.Path` of the raw
-file and should return `pathlib.Path` of the preprocessed file or folder.
+In general, we are ok with small performance hits of iterating archives rather than their extracted content. However, if
+the performance hit becomes significant, the archives can still be decompressed or extracted. To do this, the
+`decompress: bool` and `extract: bool` flags can be used for every `OnlineResource` individually. For more complex
+cases, each resource also accepts a `preprocess` callable that gets passed a `pathlib.Path` of the raw file and should
+return `pathlib.Path` of the preprocessed file or folder.