According to the [official TOSA
spec](https://www.mlplatform.org/tosa/tosa_spec.html#_cast), `tosa.cast`
allows a cast from `fp32` to `fp16`. We were not previously accounting
for this in the `TorchToTosa` lowering.
Also did a tiny bit of cleanup in the code to make it easier to spot
which conversions are currently allowed.
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Co-authored-by: Srinath Avadhanula <srinath.avadhanula@getcruise.com>
Srinath Avadhanula