[TF Bart] Refactor TFBart (#9029)
* reorder file
* delete unnecesarry function
* make style
* save intermediate
* fix attention masks
* correct tf bart past key values
* solve merge conflict bug
* correct tensor dims
* save intermediate tf
* change attn layer
* fix typo re-order past
* inputs_embeds
* make fix copies
* finish tests
* fix graph mode
* appyl lysandres suggestions
