Fix torch_dtype warning showing wrong value in from_single_file

[Ricardo-M-L]

What does this PR do?

In FromSingleFileMixin.from_single_file(), when an invalid torch_dtype is passed, the warning message always displays torch.float32 instead of the actual invalid value:

# Before (bug):
if torch_dtype is not None and not isinstance(torch_dtype, torch.dtype):
    torch_dtype = torch.float32  # reassigned BEFORE the warning
    logger.warning(
        f"Passed `torch_dtype` {torch_dtype} is not a `torch.dtype`. ..."
        # ^ always shows "torch.float32"
    )

This PR moves the reassignment after the warning so users can see what invalid value they actually passed:

# After (fix):
if torch_dtype is not None and not isinstance(torch_dtype, torch.dtype):
    logger.warning(
        f"Passed `torch_dtype` {torch_dtype} is not a `torch.dtype`. ..."
        # ^ now shows the actual invalid value (e.g. "float16", "bf16", etc.)
    )
    torch_dtype = torch.float32

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?

Who can review?

@yiyixuxu @sayakpaul