"# Unless required by applicable law or agreed to in writing, software\n",
"# distributed under the License is distributed on an \"AS IS\" BASIS,\n",
"# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.\n",
"# See the License for the specific language governing permissions and\n",
"# limitations under the License."
]
},
{
"cell_type": "markdown",
"metadata": {
"id": "fsACVQpVSifi"
},
"source": [
"### Install the TensorFlow Model Garden pip package\n",
"\n",
"* `tf-models-official` is the stable Model Garden package. Note that it may not include the latest changes in the `tensorflow_models` github repo. To include latest changes, you may install `tf-models-nightly`,\n",
"which is the nightly Model Garden package created daily automatically.\n",
"* pip will install all models and dependencies automatically."
" \u003ca target=\"_blank\" href=\"https://www.tensorflow.org/official_models/tutorials/decoding_api_in_tf_nlp.ipynb\"\u003e\u003cimg src=\"https://www.tensorflow.org/images/tf_logo_32px.png\" /\u003eView on TensorFlow.org\u003c/a\u003e\n",
" \u003c/td\u003e\n",
" \u003ctd\u003e\n",
" \u003ca target=\"_blank\" href=\"https://colab.research.google.com/github/tensorflow/models/blob/master/official/colab/decoding_api_in_tf_nlp.ipynb\"\u003e\u003cimg src=\"https://www.tensorflow.org/images/colab_logo_32px.png\" /\u003eRun in Google Colab\u003c/a\u003e\n",
" \u003c/td\u003e\n",
" \u003ctd\u003e\n",
" \u003ca target=\"_blank\" href=\"https://github.com/tensorflow/models/blob/master/official/colab/decoding_api_in_tf_nlp.ipynb\"\u003e\u003cimg src=\"https://www.tensorflow.org/images/GitHub-Mark-32px.png\" /\u003eView source on GitHub\u003c/a\u003e\n",
" This implementation returns the top logits based on probabilities.\n",
"\n",
"2. Beam search is provided in beam_search.py. [github](https://github.com/tensorflow/models/blob/master/official/nlp/modeling/ops/beam_search.py)\n",
"\n",
" This implementation reduces the risk of missing hidden high probability logits by keeping the most likely num_beams of logits at each time step and eventually choosing the logits that has the overall highest probability."
]
},
{
"cell_type": "markdown",
"metadata": {
"id": "MfOj7oaBRQnS"
},
"source": [
"## Initialize Sampling Module in TF-NLP.\n",
"\n",
"\n",
"\u003e **symbols_to_logits_fn** : This is a closure implemented by the users of the API. The input to this closure will be \n",
"```\n",
"Args:\n",
" 1] ids [batch_size, .. (index + 1 or 1 if padded_decode is True)],\n",
" 2] index [scalar] : current decoded step,\n",
" 3] cache [nested dictionary of tensors].\n",
"Returns:\n",
" 1] tensor for next-step logits [batch_size, vocab]\n",
" 2] the updated_cache [nested dictionary of tensors].\n",
"```\n",
"This closure calls the model to predict the logits for the 'index+1' step. The cache is used for faster decoding.\n",
"Here is a [reference](https://github.com/tensorflow/models/blob/master/official/nlp/modeling/ops/beam_search_test.py#L88) implementation for the above closure.\n",
"\n",
"\n",
"\u003e **length_normalization_fn** : Closure for returning length normalization parameter.\n",
"\u003e **max_decode_length** : Scalar for total number of decoding steps.\n",
"\n",
"\u003e **eos_id** : Decoding will stop if all output decoded ids in the batch have this ID.\n",
"\n",
"\u003e **padded_decode** : Set this to True if running on TPU. Tensors are padded to max_decoding_length if this is True.\n",
"\n",
"\u003e **top_k** : top_k is enabled if this value is \u003e 1.\n",
"\n",
"\u003e **top_p** : top_p is enabled if this value is \u003e 0 and \u003c 1.0\n",
"\n",
"\u003e **sampling_temperature** : This is used to re-estimate the softmax output. Temperature skews the distribution towards high probability tokens and lowers the mass in tail distribution. Value has to be positive. Low temperature is equivalent to greedy and makes the distribution sharper, while high temperature makes it more flat.\n",
"\n",
"\u003e **enable_greedy** : By default, this is true and greedy decoding is enabled.\n"
]
},
{
"cell_type": "markdown",
"metadata": {
"id": "lV1RRp6ihnGX"
},
"source": [
"# Initialize the Model Hyper-parameters"
]
},
{
"cell_type": "code",
"execution_count": null,
"metadata": {
"id": "eTsGp2gaKLdE"
},
"outputs": [],
"source": [
"params = {}\n",
"params['num_heads'] = 2\n",
"params['num_layers'] = 2\n",
"params['batch_size'] = 2\n",
"params['n_dims'] = 256\n",
"params['max_decode_length'] = 4"
]
},
{
"cell_type": "markdown",
"metadata": {
"id": "UGvmd0_dRFYI"
},
"source": [
"## What is a Cache?\n",
"In auto-regressive architectures like Transformer based [Encoder-Decoder](https://arxiv.org/abs/1706.03762) models, \n",
"Cache is used for fast sequential decoding.\n",
"It is a nested dictionary storing pre-computed hidden-states (key and values in the self-attention blocks and in the cross-attention blocks) for every layer.\n",
" In practice, this will be replaced by an actual model implementation such as [here](https://github.com/tensorflow/models/blob/master/official/nlp/transformer/transformer.py#L236)\n",
"```\n",
"Args:\n",
"i : Step that is being decoded.\n",
"Returns:\n",
" logit probabilities of size [batch_size, 1, vocab_size]\n",
"Greedy decoding selects the token id with the highest probability as its next id: $id_t = argmax_{w}P(id | id_{1:t-1})$ at each timestep $t$. The following sketch shows greedy decoding. "
"Instead of sampling only from the most likely *K* token ids, in *Top-p* sampling chooses from the smallest possible set of ids whose cumulative probability exceeds the probability *p*."
"Beam search reduces the risk of missing hidden high probability token ids by keeping the most likely num_beams of hypotheses at each time step and eventually choosing the hypothesis that has the overall highest probability. "
