fix get_keys_to_not_convert function
# What does this PR do ? 

Fix the behavior of the get_keys_to_not_convert function for the following cases: 
- If the lm_head is tied, we won't be able to see it using the method named_parameters() and  the last visible module was added instead -> using `named_children() `instead
- Fix tied_params variable that we should not crop.(  Example of what was happening : `[['lm_head.weight', 'model.decoder.embed_tokens.weight']] -> ['model.decoder.embed_tokens.weight']`)


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