[WIP] Add Pegasus
* Add PEGASUS in TF2
Hi @sshleifer, I will continue to push code to this PR and make it runnable asap.<|||||>@sshleifer The code has been uploaded. (1) The test is runnable in TF2 and loads AESLC checkpoints successfully with correct outputs. (2) Code of models and layers (including decoding, beam search) are all in a single file which may look messy (sorry). (3) Most code is simply copied and pasted (then converted to TF2) from the original PEGASUS repo so you may refer to the original repo for clearer code if necessary. 

I think you can start from here. Please let me know if I can help further.<|||||>@JingqingZ I'm gunna add torch first in #6340 (you will be a PR co-author). And then come back here to finish TF. No action needed from you, just an update.<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
