Compute loss independent from decoder for TF EncDec models (as #14139)
# What does this PR do?

Apply the same change in #14139 to TF Encoder-Decoder like models.

(As discussed in [this comment in 14148](https://github.com/huggingface/transformers/pull/14148#discussion_r780361117))

@NielsRogge & @patrickvonplaten  knows better the context
@gante @Rocketknight1 for TF