Add shift-right method to T5 model

[yohann-benchetrit]

We add a _shift_right method to the T5 Model. This is used for training (see Issue 2127).

It shifts an input batch of sequences of token-indices, one index to the right. Their first token is replaced with the start-of-sequence token, which is the padding-token index for T5 models

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[joecummings]

@yohann-benchetrit Can you add a unittest for this?

[joecummings]

@yohann-benchetrit Looks like there's an error in how the config is being called within _shift_right