Support named tuple return from operators on JIT (#16253)

Support named tuple return from operators on JIT (#16253)

Summary:
Fixes: https://github.com/pytorch/pytorch/issues/16233

The following changes are made:
- Modify `TupleType` to store optional field names
- Modify schema matching to return fill in those field names when creating  `TupleType` as return type.
- Modify codegen of JIT to copy field names to schema string
- Modify `SchemaParser` to set field names of returned schema.
- Modify `SimpleValue::attr` to emit tuple indexing for named tuple.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16253

Reviewed By: ezyang

Differential Revision: D13954298

Pulled By: zdevito

fbshipit-source-id: 247d483d78a0c9c12d1ba36e1f1ec6c3f1a3007b