Update on "[ET-VK] Serialize constant data outside of flatbuffer"

## Context

This changeset is a mirror of #1542 and #1543 for the Vulkan delegate, which enables serialization of  constant weight data outside the flatbuffer blob.

I decided to package some miscellaneous codebase improvements with this changeset:

* Update field names of `VkGraph` table in `schema.fbs` and update the namespace to be simply `vkgraph`:
* In `VulkanBackend.cpp`, use aliases for generated flatbuffer types
* Move build targets to a `targets.bzl` file with a `define_common_targets()` method

Differential Revision: [D53957852](https://our.internmc.facebook.com/intern/diff/D53957852/)

[ghstack-poisoned]

Author: SS-JIA

backends/vulkan/test/test_serialization.py

@@ -36,7 +36,7 @@ def _generate_random_const_tensors(self, num_tensors: int) -> List[torch.Tensor]
 
         return tensors
 
-    def test_serialize_xnnpack_binary(self):
+    def test_serialize_vulkan_binary(self):
         vk_graph = VkGraph(
             version="0",
             chain=[],

backends/vulkan/vulkan_preprocess.py

@@ -4,7 +4,6 @@
 # This source code is licensed under the BSD-style license found in the
 # LICENSE file in the root directory of this source tree.
 
-import ctypes
 from typing import Dict, final, List
 
 import executorch.backends.vulkan.serialization.vulkan_graph_schema as vk_graph_schema
@@ -178,8 +177,6 @@ def create_vk_values_for(node: Node, buffer_idx: int = -1):
                     ),
                 )
             elif node.op == "get_attr":
-                # Tensor
-                # Adapted from https://www.internalfb.com/code/fbsource/[18c174b709f321d26e6632e2f826498cde730f8c]/fbcode/executorch/backends/xnnpack/xnnpack_preprocess.py?lines=127
                 buffer_idx = len(const_tensors)
                 const_tensors.append(
                     getattr(node.graph.owning_module, node.target).contiguous()