Add loss for BridgeTowerForMaskedLM and BridgeTowerForImageAndTextRetrieval
Author: abhiwand
Pull request: yes

# What does this PR do?
This PR adds losses to BridgeTowerMaskedLM and BridgeTowerForImageAndTextRetrieval
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Fixes # (issue)


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Comments:
_The documentation is not available anymore as the PR was closed or merged._

cc @amyeroberts and @younesbelkada 

Thank @amyeroberts, @younesbelkada, @regisss for your review and your suggestions. We have addressed your comments and have added few tests for loss computation and for forward/backward as suggested ones. 
Can you please help to merge this PR if possible? Thanks a lot

Thank @amyeroberts for approving this PR.
Thank @younesbelkada for your suggestion. I have resolved the failed quality tests. Can you please approve and merge this PR? c
Thanks a lot

> Let's see what @ydshieh & @sgugger will say

No problem for me, as this is used in > 100 places, and the tensor is changed to a python scalar before using it.