TF: `unpack_inputs` decorator independent from `main_input_name`
# What does this PR do?

As the title indicates -- the `unpack_inputs` decorator becomes independent from `main_input_name` in this PR.

The old `input_processing` included some checks for `input_ids`, which somewhat transitioned into the `unpack_inputs` decorator (the `input_ids` input was obtained there from the argument under `main_input_name`). However, in practice, it is not needed -- what we want is to support the case where all model arguments come packed in the first input, which happens to be `main_input_name` for most use cases of `unpack_inputs`. Note that Keras often expects this packing behavior with input dictionaries, which `input_processing` maps back to our expected format.

Fixes #18040
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