Add IOBuf support

Summary: Currently if we have `cpp.Type{name = "folly::IOBuf"}`, the python type of the field will be changed to IOBuf. In this case we can not assign `bytes` to IOBuf field directly (otherwise there will be runtime error due to type mismatch).

Reviewed By: thedavekwon

Differential Revision: D64764837

fbshipit-source-id: 4b9e5c73a954e9bdef6d674867cbbe67c15649fe

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

@@ -41,6 +41,7 @@ from common.thrift.patch.detail.py_bindings.DynamicPatch import (
 )
 
 import thrift.python.types as _fbthrift_python_types
+import folly.iobuf as _fbthrift_iobuf
 
 import {{program:module_path}}.thrift_types
 from {{program:module_path}}.thrift_types import (

third-party/thrift/src/thrift/compiler/test/fixtures/basic/out/python_patch/gen-python-patch/thrift_patch.py

@@ -24,6 +24,7 @@
 )
 
 import thrift.python.types as _fbthrift_python_types
+import folly.iobuf as _fbthrift_iobuf
 
 import test.fixtures.basic.module.thrift_types
 from test.fixtures.basic.module.thrift_types import (