QA pipeline BART compatible (#5496)

* Ensure padding and question cannot have higher probs than context.

Signed-off-by: Morgan Funtowicz <funtowiczmo@gmail.com>

* Add bart the the list of tokenizers adding two <sep> tokens for squad_convert_example_to_feature

Signed-off-by: Morgan Funtowicz <funtowiczmo@gmail.com>

* Format.

Signed-off-by: Morgan Funtowicz <funtowiczmo@gmail.com>

* Addressing @patrickvonplaten comments.

Signed-off-by: Morgan Funtowicz <funtowiczmo@gmail.com>

* Addressing @patrickvonplaten comments about masking non-context element when generating the answer.

Signed-off-by: Morgan Funtowicz <funtowiczmo@gmail.com>

* Addressing @sshleifer comments.

Signed-off-by: Morgan Funtowicz <funtowiczmo@gmail.com>

* Make sure we mask CLS after handling impossible answers

Signed-off-by: Morgan Funtowicz <funtowiczmo@gmail.com>

* Mask in the correct vectors ...

Signed-off-by: Morgan Funtowicz <funtowiczmo@gmail.com>

src/transformers/data/processors/squad.py

@@ -12,6 +12,10 @@ from ...tokenization_bert import whitespace_tokenize
 from .utils import DataProcessor
 
 
+# Store the tokenizers which insert 2 separators tokens
+MULTI_SEP_TOKENS_TOKENIZERS_SET = {"roberta", "camembert", "bart"}
+
+
 if is_torch_available():
     import torch
     from torch.utils.data import TensorDataset
@@ -123,9 +127,13 @@ def squad_convert_example_to_features(example, max_seq_length, doc_stride, max_q
     truncated_query = tokenizer.encode(
         example.question_text, add_special_tokens=False, truncation=True, max_length=max_query_length
     )
+
+    # Tokenizers who insert 2 SEP tokens in-between <context> & <question> need to have special handling
+    # in the way they compute mask of added tokens.
+    tokenizer_type = type(tokenizer).__name__.replace("Tokenizer", "").lower()
     sequence_added_tokens = (
         tokenizer.max_len - tokenizer.max_len_single_sentence + 1
-        if "roberta" in str(type(tokenizer)) or "camembert" in str(type(tokenizer))
+        if tokenizer_type in MULTI_SEP_TOKENS_TOKENIZERS_SET
         else tokenizer.max_len - tokenizer.max_len_single_sentence
     )
     sequence_pair_added_tokens = tokenizer.max_len - tokenizer.max_len_sentences_pair