[Bart: example] drop columns that are exclusively pad_token_id… (#3400)
[Bart: example] drop columns that are exclusively pad_token_id… (#3400)

* trim seq_len below 1024 if there are columns full of pad_token_id
* Centralize trim_batch so SummarizationDataset can use it too
huggingface/transformers,huggingface/pytorch-transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers