TF MT5 embeddings resize
# What does this PR do?

Fixes #13839 

In MT5, the `get_output_embeddings()` method does not return a layer, returns a `tf.Tensor` with the weights. As such, `self._get_word_embedding_weight(self.get_output_embeddings())` was failing, as it expected a layer. Since the point of `_get_word_embedding_weight` is to get the weights, added a return for the case where we already have the weights. Also added tests to ensure we don't regress.
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