aten.pow.Tensor_Tensor (#1880)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1880

This wires the eager-mode operation to the Vulkan shader. We only cover the case where both inputs are Tensor type, which is on par with the existing operators: add, sub, mul, div, floor_div.

It doesn't seem like we can cover [any other of the 8 cases](https://www.internalfb.com/code/fbsource/[e45c04564445b5e67ebb61e6ba53995729686526]/xplat/caffe2/torch/distributed/_tensor/ops/pointwise_ops.py?lines=310-317), right now. We categorize them and explain that what's missing for each.

## Category 1
The other 2/3 "standard" cases requires one of the values to be a scalar,
```
z = torch.pow(x, y)
```
```
aten.pow.Scalar,
aten.pow.Tensor_Scalar,
aten.pow.Tensor_Tensor,
```
which is not currently supported.
```
F 00:00:01.746228 executorch:aten_bridge.cpp:21] In function check_tensor_meta(), assert failed (b.sizes().data() != nullptr): ETensor must have valid sizes array
```

## Category 2
IIUC, these operators require an out argument in the declaration. However, when they are traced they collapsed into Category 1, e.g., we obtain `aten.pow.Tensor_Tensor` not `aten.pow.Tensor_Tensor_out`.

This appears in line with current PT-Vulkan, which only [implements the other two categories](https://www.internalfb.com/code/fbsource/[f148c22604b8e409696fd64f814cda89d091fe7a]/xplat/caffe2/aten/src/ATen/native/vulkan/ops/BinaryOp.cpp?lines=533-558).
```
torch.pow(x, y, out=z)
```
```
aten.pow.Scalar_out,
aten.pow.Tensor_Scalar_out,
aten.pow.Tensor_Tensor_out,
```

## Category 3
IIUC, in-place operators are written like this:
```
x.pow_(y)
```
```
aten.pow_.Scalar,
aten.pow_.Tensor,
```
They are not currently supported.
```
  File "/data/users/jorgep31415/fbsource/buck-out/v2/gen/fbcode/b007eb344207ad7d/executorch/backends/vulkan/test/__test_vulkan_delegate__/test_vulkan_delegate#link-tree/torch/_export/verifier.py", line 188, in _check_valid_op
    raise SpecViolationError(
torch._export.verifier.SpecViolationError: operator 'aten.copy_.default' is not functional
```

Reviewed By: yipjustin

Differential Revision: D53547865

fbshipit-source-id: 777afafd8c2614a2e5b3cfbc7e046ea3864ef57c