Fix usage of head masks by TF encoder-decoder models' `generate()` function
TF counterpart to #11621 

**Description:** It is necessary to fix head masking for LED and T5 models.

Edit: Fix for T5 - #11857

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**Reviewers:** @patrickvonplaten @Rocketknight1 
Thanks for the addition @stancld ! 

I think once we fix the tests in led + T5 we can merge this one :-)<|||||>It also looks good to me!<|||||>Hey @patrickvonplaten, I haven't implemented head masking for the `generate` method for LED and T5 intentionally. The reason is that TF LED and T5 does not use head masks properly (there's an old glitch that the decoder uses encoder's `head_mask` instead of `cross_attn_head_mask`).  Maybe, I can fix this issue in other PRs and then enable testing for these two models? :)<|||||>> Hey @patrickvonplaten, I haven't implemented head masking for the `generate` method for LED and T5 intentionally. The reason is that TF LED and T5 does not use head masks properly (there's an old glitch that the decoder uses encoder's `head_mask` instead of `cross_attn_head_mask`). Maybe, I can fix this issue in other PRs and then enable testing for these two models? :)

Good for me!