Make sure `'torch.dtype'` has str-type value in config and all nested dicts for JSON serializability 
Author: feifang24
Pull request: yes

# What does this PR do?
Currently a model config is not JSON serializable if 
- it contains nested dicts, 
- those dicts have `'torch.dtype'` as a key,
- the value is of type `torch.dtype` rather than `str`

The changes check for `'torch.dtype'` in the nested dicts and convert them to `str` if necessary.

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Fixes #16054.


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