[Generation, EncoderDecoder] Apply Encoder Decoder 1.5GB memory savings to TF as well
As was done by @sshleifer for torch, improved the memory usage for TF Encoder Decoder models. 
Straight-forward translation of PR: https://github.com/huggingface/transformers/pull/3370.
Tested on `RUN SLOW=1 pytest tests/test_modeling_tf_t5.py` and all tests pass.