Update pretrained_models.rst
# What does this PR do?

Updates the description of facebook/bart-base and facebook/bart-large in Pretrained models to specify the number of encoder and decoder layers according to #11574 
Hi @patil-suraj , 
I added the (N encoder and decoder layers) to the existing descriptions for facebook/bart-base and facebook/bart-large. Thanks!<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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