remove BartForConditionalGeneration.generate (#6659)
remove BartForConditionalGeneration.generate (#6659)

As suggested here: https://github.com/huggingface/transformers/issues/6651#issuecomment-678594233
this removes generic `generate` doc with examples not-relevant to bart.
huggingface/transformers,huggingface/pytorch-transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers