Make get_last_lr in trainer backward compatible (#4446)
Make get_last_lr in trainer backward compatible (#4446)

* makes fetching last learning late in trainer backward compatible

* split comment to multiple lines

* fixes black styling issue

* uses version to create a more explicit logic
huggingface/transformers,huggingface/transformers,huggingface/transformers,huggingface/pytorch-transformers,huggingface/transformers