Change assertion to warning when passing past_key_value to T5 encoder
Author: ZhaofengWu
Pull request: yes

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
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- tokenizers: @n1t0, @LysandreJik
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Comments:
_The documentation is not available anymore as the PR was closed or merged._

@patrickvonplaten 

I'm fine with this change

Thanks. I don't think I can merge -- please merge whenever makes sense.

@patil-suraj feel free to merge if ok for you

hi guys, I have one question, the `T5Encoder` is only used when the `encoder_outputs` is None, so what's the meaning for the `T5Encoder`.