Pass output options to FLAVA multimodal transformer block
Author: KhoomeiK
Pull request: yes

# What does this PR do?
Currently, the FLAVA model passes `output_attentions` and `output_hidden_states` only to its text & image blocks, preventing external access to cross-modal attentions in the multimodal block. This simple PR adds this ability by passing the `output_attentions` and `output_hidden_states` options to `FlavaMultimodalModel` during the `forward` pass.

I personally need to access these cross-modal attentions for implementing an auxiliary loss ([IAIS](https://github.com/lancopku/IAIS/)) which uses them, and this PR doesn't significantly change the behavior of this model.

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Comments:
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