Update on "[ET-VK] Introduce AOT operator registry"

## Changes

Move the following files to the root directory of Vulkan backend:

* `backends/vulkan/partitioner/supported_ops.py` -> `backends/vulkan/op_registry.py`
* `backends/vulkan/_passes/custom_ops_defs.py` -> `backends/vulkan/custom_ops_lib.py`

In the new `op_registry.py` file, the way operator features are specified is reworked to provide much more detail about the features of the operator implementation in Vulkan. See the new `OpFeatures` class for more details. An example of registering a new operator to the export flow is

```
update_features(
    [
        exir_ops.edge.aten._log_softmax.default,
        exir_ops.edge.aten._softmax.default,
        exir_ops.edge.aten.mean.dim,
        exir_ops.edge.aten.sum.dim_IntList,
        exir_ops.edge.aten.amax.default,
        exir_ops.edge.aten.amin.default,
    ]
)
def register_reduce_op(features: OpFeatures):
    features.texture_impl = TextureImplFeatures(
        uses_packed_dim=True,
    )
    features.resize_fn = True

    def check_reduce_node(node: torch.fx.Node) -> bool:
        dim_list = node.args[1]
        assert isinstance(dim_list, list)
        if len(dim_list) != 1:
            return False

        keepdim = node.args[2]
        assert isinstance(keepdim, bool)
        if not keepdim:
            return False

        return True

    features.check_node_fn = check_reduce_node
    return features
```

## Rationale

The purpose of these changes is to centralize operator definitions so that there is a common source of truth about the capabilities of operator implementation in Vulkan. This way, the partitioner does not have to implement ad-hoc functions for specific operators (i.e. `is_valid_to_copy`) and graph transforms do not have to maintain their own operator metadata (`USES_WEIGHTS` in `insert_prepack_nodes`).

Differential Revision: [D64915640](https://our.internmc.facebook.com/intern/diff/D64915640/)

[ghstack-poisoned]

Author: SS-JIA