Update on "[ET-VK][7/n] Slice, with lots of codegen improvements"

1. Add slice operation. Instead of using copy in LI, we implement a simple shader with offsets.

2. Improvement in codegen.
- add support of optional variables
- improve indent of the code, for better readability
- allow user to specify tensor value generation, possible to generate sequential values for easier debugging for index operations
- sample code improve test-case specification, particularly with long and optional values.

Differential Revision: [D56295985](https://our.internmc.facebook.com/intern/diff/D56295985/)

[ghstack-poisoned]

Author: yipjustin

backends/vulkan/test/op_tests/utils/codegen_base.py

@@ -117,7 +117,7 @@ def gen_case_name(self, inputs: List[Any], prepack: bool = False) -> str:
         name_str = name_str.replace("-", "n")
         return name_str
 
-    def create_input_data(self, arg: Argument, data: Any) -> str:
+    def create_input_data(self, arg: Argument, data: Any) -> str:  # noqa: C901
         ctype = cpp.argumenttype_type(arg.type, mutable=arg.is_write, binds=arg.name)
         cpp_type = ctype.cpp_type(strip_ref=True)