Remove file-scope non-constant static variables to support multiple inference sessions

[satyajandhyala]

Description: Describe your changes.
Remove non-constant file-scope static ("global static") variables.
The scope of this PR is only to enable multiple concurrent InferenceSession instances by removing non-constant static variables in the file-scope.
Motivation and Context
Make multiple concurrent InferenceSessions possible by encapsulating data in the class members.

• Why is this change required? What problem does it solve?
  Enable using multiple inference sessions.
• If it fixes an open issue, please link to the issue here.
  PropagateCastOps uses and modifies 3 global containers at runtime #10473

[edgchen1]

nit: lines below are kind of long.
if this is the only place that level1_fp16_allow_ops/level2_fp16_allow_ops are used, how about just initializing them here?

Suggested change

	: GraphTransformer("PropagateCastOps", compatible_execution_providers), level_(level), fp16_allow_ops_(3), strategy_(strategy) {
	: GraphTransformer("PropagateCastOps", compatible_execution_providers), level_(level),
	fp16_allow_ops_{
	std::unordered_set<std::string>(allow_list.begin(), allow_list.end()),
	{"level1 op", ... },
	{"level2 op", ... }
	},
	strategy_(strategy) {

and then fp16_allow_ops_ can be const too

[satyajandhyala]

I want to keep the level1 and level2 operators at the top of the file so that it is easy to modify (add new operators and delete existings ones.) and/or add a new levels of fp16 operator all together.

[satyajandhyala]

FP16AllowOps is designed to make adding new operator and new levels easy, i.e. to add new levels or new operators without modifying the code.

[edgchen1]

if you want to keep them separate, it's fine too.

constexpr std::array level1_fp16_allow_ops = {"...", ... }; // deduced to std::array<const char*, N>, don't need to specify N
...
  // ctor member initializer list
  fp16_allow_ops_{
      std::unordered_set<std::string>(allow_list.begin(), allow_list.end()),
      std::unordered_set<std::string>(level1_fp16_allow_ops.begin(), level1_fp16_allow_ops.end()),
      std::unordered_set<std::string>(level2_fp16_allow_ops.begin(), level2_fp16_allow_ops.end())
  }

as is, the lines in the ctor are longer than 120 characters

[satyajandhyala]

Making the suggested change resulted in the following error:
/home/sajandhy/onnxruntime/onnxruntime/core/optimizer/propagate_cast_ops.cc:1468:269: required from here
/usr/include/c++/9/bits/hashtable.h:994:4: error: no matching function for call to ‘std::_Hashtable<std::__cxx11::basic_string, std::__cxx11::basic_string, std::allocator<std::__cxx11::basic_string >, std::__detail::_Identity, std::equal_to<std::__cxx11::basic_string >, std::hash<std::__cxx11::basic_string >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, true, true> >::insert(const std::basic_string_view&)’

[satyajandhyala]

because it requires conversion from string_view to string.