Set _torch_version to N/A if torch is disabled. #11645
                
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What does this PR do?
Set a default value for _torch_version if torch is disabled. This fixes an ImportError in hub_utils.py when it tries to import _torch_version. This is not riased for Jax and Flax because they already set
_jax_version = "N/A"as a default.Who can review?
Core library: