Use acc dtype if out_dtype is not specified for tl.dot

[mrTsjolder]

This solves issue #10341

When specifying the acc argument of tl.dot with dtype tl.float16 without out_dtype=tl.float16 raises an AssertionError because the default for out_dtype is hard-coded to be tl.float32.
This PR changes the default out_dtype to be None to allow checking whether a value has been provided.
This again allows to use the dtype of acc as the default for out_dtype (if the acc argument is specified).

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