feat: support masked_scatter by lowering path

[chohk88]

Description

Implemented support for masked_scatter in the lowering path, referring to this implementation in PyTorch Inductor.

Fixes # (issue)

Type of change

Please delete options that are not relevant and/or add your own.

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines of this project (You can use the linters)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas and hacks
• I have made corresponding changes to the documentation
• I have added tests to verify my fix or my feature
• New and existing unit tests pass locally with my changes
• I have added the relevant labels to my PR in so that relevant reviewers are notified

[peri044]

@chohk88 I have a question. I tried running this code in a separate python session for the test input size of (2, 3, 4) and I see the following error. Do you know why this happens ? Am I missing something here ?

import torch
shape=(2, 3, 4)

ax=torch.randn(*shape, dtype=torch.float32, device="cuda")
mask=torch.rand(*shape, device="cuda") > 0.5
num_trues = mask.sum().item()
source = torch.arange(num_trues, dtype=torch.float32, device="cuda")
ax_b, mask_b = torch.ops.aten.broadcast_tensors([ax, mask])
ax_flat = ax_b.flatten()
mask_flat = mask_b.flatten()
source_flat = source.flatten()
source_idx = mask_flat.to(torch.int64).cumsum(0) - 1
gathered = source_flat.gather(0, source_idx)
>>> /pytorch/aten/src/ATen/native/cuda/ScatterGatherKernel.cu:144: operator(): block: [0,0,0], thread: [0,0,0] Assertion `idx_dim >= 0 && idx_dim < index_size && "index out of bounds"` failed.
/pytorch/aten/src/ATen/native/cuda/ScatterGatherKernel.cu:144: operator(): block: [0,0,0], thread: [1,0,0] Assertion `idx_dim >= 0 && idx_dim < index_size && "index out of bounds"` failed.