From 1c7253cc5f233978ddf6804efc71c0358acbac45 Mon Sep 17 00:00:00 2001
From: Stefan Schweter <stefan.schweter@bsb-muenchen.de>
Date: Mon, 11 Nov 2019 16:18:16 +0100
Subject: [PATCH] modeling: add DistilBertForTokenClassification implementation

---
 transformers/modeling_distilbert.py | 73 +++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)

diff --git a/transformers/modeling_distilbert.py b/transformers/modeling_distilbert.py
index 00106627a8..d30f493c69 100644
--- a/transformers/modeling_distilbert.py
+++ b/transformers/modeling_distilbert.py
@@ -30,6 +30,7 @@ import numpy as np
 
 import torch
 import torch.nn as nn
+from torch.nn import CrossEntropyLoss
 
 from .modeling_utils import PreTrainedModel, prune_linear_layer
 from .configuration_distilbert import DistilBertConfig
@@ -702,3 +703,75 @@ class DistilBertForQuestionAnswering(DistilBertPreTrainedModel):
             outputs = (total_loss,) + outputs
 
         return outputs  # (loss), start_logits, end_logits, (hidden_states), (attentions)
+
+
+@add_start_docstrings("""DistilBert Model with a token classification head on top (a linear layer on top of
+                      the hidden-states output) e.g. for Named-Entity-Recognition (NER) tasks. """,
+                      DISTILBERT_START_DOCSTRING,
+                      DISTILBERT_INPUTS_DOCSTRING)
+class DistilBertForTokenClassification(DistilBertPreTrainedModel):
+    r"""
+        **labels**: (`optional`) ``torch.LongTensor`` of shape ``(batch_size, sequence_length)``:
+            Labels for computing the token classification loss.
+            Indices should be in ``[0, ..., config.num_labels - 1]``.
+
+    Outputs: `Tuple` comprising various elements depending on the configuration (config) and inputs:
+        **loss**: (`optional`, returned when ``labels`` is provided) ``torch.FloatTensor`` of shape ``(1,)``:
+            Classification loss.
+        **scores**: ``torch.FloatTensor`` of shape ``(batch_size, sequence_length, config.num_labels)``
+            Classification scores (before SoftMax).
+        **hidden_states**: (`optional`, returned when ``config.output_hidden_states=True``)
+            list of ``torch.FloatTensor`` (one for the output of each layer + the output of the embeddings)
+            of shape ``(batch_size, sequence_length, hidden_size)``:
+            Hidden-states of the model at the output of each layer plus the initial embedding outputs.
+        **attentions**: (`optional`, returned when ``config.output_attentions=True``)
+            list of ``torch.FloatTensor`` (one for each layer) of shape ``(batch_size, num_heads, sequence_length, sequence_length)``:
+            Attentions weights after the attention softmax, used to compute the weighted average in the self-attention heads.
+
+    Examples::
+
+        tokenizer = DistilBertTokenizer.from_pretrained('distilbert-base-uncased')
+        model = DistilBertForTokenClassification.from_pretrained('distilbert-base-uncased')
+        input_ids = torch.tensor(tokenizer.encode("Hello, my dog is cute")).unsqueeze(0)  # Batch size 1
+        labels = torch.tensor([1] * input_ids.size(1)).unsqueeze(0)  # Batch size 1
+        outputs = model(input_ids, labels=labels)
+        loss, scores = outputs[:2]
+
+    """
+    def __init__(self, config):
+        super(DistilBertForTokenClassification, self).__init__(config)
+        self.num_labels = config.num_labels
+
+        self.distilbert = DistilBertModel(config)
+        self.dropout = nn.Dropout(config.dropout)
+        self.classifier = nn.Linear(config.hidden_size, config.num_labels)
+
+        self.init_weights()
+
+    def forward(self, input_ids=None, attention_mask=None, head_mask=None,
+                inputs_embeds=None, labels=None):
+
+        outputs = self.distilbert(input_ids,
+                            attention_mask=attention_mask,
+                            head_mask=head_mask,
+                            inputs_embeds=inputs_embeds)
+
+        sequence_output = outputs[0]
+
+        sequence_output = self.dropout(sequence_output)
+        logits = self.classifier(sequence_output)
+
+        outputs = (logits,) + outputs[2:]  # add hidden states and attention if they are here
+        if labels is not None:
+            loss_fct = CrossEntropyLoss()
+            # Only keep active parts of the loss
+            if attention_mask is not None:
+                active_loss = attention_mask.view(-1) == 1
+                active_logits = logits.view(-1, self.num_labels)[active_loss]
+                active_labels = labels.view(-1)[active_loss]
+                loss = loss_fct(active_logits, active_labels)
+            else:
+                loss = loss_fct(logits.view(-1, self.num_labels), labels.view(-1))
+            outputs = (loss,) + outputs
+
+        return outputs  # (loss), scores, (hidden_states), (attentions)