Do not generate abstract property for invariant types.

Summary:
# What?
Do not generate `fbthrift_current_value` if any field of the union is an invariant type.

# Why?
`fbthrift_current_value` returns a `typing.Union` of the types of the fields in the union plus `None`. If any of the fields have an invariant type, then the type-check will report that the `fbthrift_current_value` in the derived class in `thrift_mutable_types` overrides the attribute defined in the `thrift_abstract_types` base class inconsistently. `thrift_types` doesn't flag this same error due to some unrelated error that has a separate task to fix it.

# Build fixtures
`buck2 run fbcode//thrift/compiler/test:build_fixtures`

# Internal
# What was the canary?
Instagram pyre check flagged this lone error below (P1711855823). You could say this was a needle-in-the-haystack as well.
```
distillery/igthrift/gen/apache/thrift/protocol/detail/protocol_detail/thrift_mutable_types.pyi:172:4 Inconsistent override [15]: `fbthrift_current_value` overrides attribute defined in `_fbthrift_python_abstract_types.Value` inconsistently. Type `_typing.Union[None, _fbthrift_python_mutable_containers.MutableList[Value], _fbthrift_python_mutable_containers.MutableMap[Value, Value], _fbthrift_python_mutable_containers.MutableSet[Value], Object, _fbthrift_iobuf.IOBuf, bool, bytes, float, int]` is not a subtype of the overridden attribute `_typing.Union[None, _typing.AbstractSet[_fbthrift_python_abstract_types.Value], _typing.Mapping[_fbthrift_python_abstract_types.Value, _fbthrift_python_abstract_types.Value], _typing.Sequence[_fbthrift_python_abstract_types.Value], _fbthrift_iobuf.IOBuf, _fbthrift_python_abstract_types.Object, bool, bytes, float, int]`.
```

# Task to fix `thrift_types.pyi` type checks
T212883284 [thrift-python] Fix thrift_types.pyi type-checks

Reviewed By: yoney

Differential Revision: D68270113

fbshipit-source-id: dde2d437330741239c573ecf64eadccfb44f5b86

third-party/thrift/src/thrift/compiler/generate/templates/python/thrift_abstract_types.py.mustache

@@ -91,9 +91,11 @@ class {{> structs/unadapted_name}}({{!
         used in immutable thrift-python, which is the name
         of the Union (sigh!)
     }}FbThriftUnionFieldEnum.__name__ = "{{struct:py_name}}"
+    {{^struct:has_invariant_field?}}
     @_fbthrift_property
 {{> fields/maybe_abstract_method_annotation}}
     def fbthrift_current_value(self) -> {{> types/field_value_pep484_union_type}}: ...
+    {{/struct:has_invariant_field?}}
     @_fbthrift_property
 {{> fields/maybe_abstract_method_annotation}}
     def fbthrift_current_field(self) -> FbThriftUnionFieldEnum: ...

third-party/thrift/src/thrift/compiler/test/fixtures/complex-union/out/python_invariant_types/gen-python/invariant/thrift_abstract_types.py

@@ -71,9 +71,6 @@ class FbThriftUnionFieldEnum(_enum.Enum):
     FbThriftUnionFieldEnum.__name__ = "InvariantTypes"
     @_fbthrift_property
     @_abc.abstractmethod
-    def fbthrift_current_value(self) -> _typing.Union[None, _typing.Mapping[_fbthrift_StructForInvariantTypes, int], _typing.Mapping[_fbthrift_UnionForInvariantTypes, int]]: ...
-    @_fbthrift_property
-    @_abc.abstractmethod
     def fbthrift_current_field(self) -> FbThriftUnionFieldEnum: ...
 
 _fbthrift_InvariantTypes = InvariantTypes