Fix tflongformer int dtype
TFLongformer had a lot of `int32` dtypes in its code, caused by `tf.convert_to_tensor()` defaulting to int32 when passed a list of ints, as well as some explicit `int32` lines. We prefer `int64` across our models, so I've converted everything to use that.

Fixes #13632 
_The documentation is not available anymore as the PR was closed or merged._<|||||>But it doesn't look to good to CI 😅 <|||||>Working on that bit!<|||||>Quick update: I did a lot of dtype casting which should resolve the remaining issues. Because TFLED has some sections copied from TFLongFormer, TFLED got updated as well. However, the TFLongformerEmbeddings were copied from TFRobertaEmbeddings, but I broke this connection because we have to do some extra casting in TFLongformer due to things like the `global_attention_mask`, and I don't really want to mess with TFRoberta when it doesn't have issues, because it's a heavily-used model.<|||||>Looks good! 👍 

Thanks for taking care of this one