[TOSA] : Fix float to integer cast for torch.ops.aten.to lowering.

[sahas3]

The behavior of float -> integer cast in PyTorch (though I haven't found the actual code implementing the cast) appears to be (based on the results produced in PyTorch):

1. round the float nearest to zero (similar to arith.fptosi/ui)
2. then perform the conversion

Currently we only emit tosa.cast for this operation but as per the spec https://www.mlplatform.org/tosa/tosa_spec.html#_cast the rounding performed for float -> integer is round to nearest integer (not zero). Hence, the current TOSA lowering for torch.ops.aten.to produces incorrect answer.

[justin-ngo-arm]

Hi @sahas3, for the Torch float to integer cast behavior, did you observe it to be the same for all Torch casting, or just torch.aten.to? This is because I plan to make tosaCastTensorToType the universal tosa.cast call within TorchToTosa (plan described in #3948), and if the behavior you observed is specific to aten.to then I might need to re-evaluate my approach to unifying these calls.

[sahas3]

Hi @sahas3, for the Torch float to integer cast behavior, did you observe it to be the same for all Torch casting, or just torch.aten.to? This is because I plan to make tosaCastTensorToType the universal tosa.cast call within TorchToTosa (plan described in #3948), and if the behavior you observed is specific to aten.to then I might need to re-evaluate my approach to unifying these calls.

I think torch.aten.to or torch.tensor.to ops are the basis of all casts in PyTorch. I haven't seen different behavior in my experiments yet. Some additional tests also started passing with this PR indicating that the cast behavior is same for other cast like ops too. So I think using tosaCastTensorToType universally for all tosa.cast calls is the correct approach.

[sjarus]

If Torch mandates round towards zero, can that not be synthetically expressed here @sahas3 ?

[sahas3]

If Torch mandates round towards zero, can that not be synthetically expressed here @sahas3 ?

Hi @sjarus , can you elaborate more on what you mean by synthetically expressing rounding towards zero?

[sjarus]

Looks like we had a misunderstanding earlier @sahas3 . Approving.