Fix bug in Masked Language Modeling example scripts (#11840))
# What does this PR do?
when `data_args.line_by_line == False`, the script firstly converts given examples into input_ids, token_type_ids, attention_mask and special_tokens_mask including cls_token, sep_token. Then it concatenates all tokenized outputs and generate chunks of max_seq_length. However, it will generate unintended training examples such as [871, 512, 2492, 1111, 947, 533] not [2 (cls_token), 512, 2492, 1111, 947, 3 (sep_token)]. I fix this problem.

Fixes #11840 
@sgugger, @patil-suraj
The `line_by_line=False` argument is not supposed to be used for BERT-like pretraining objectives, it it there to do GPT-like pretraining. Maybe it does not make sense to have it in `run_mlm` at all.
In any case this fix will not necessarily work for all models supported by the script, as the special tokens may be slightly different than what is hard-coded.<|||||>#10737 <|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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