Fix various code style and build issues in stream_executor/rocm
Fix various code style and build issues in stream_executor/rocm

- Adding namespace to `string` -> `std::string` (Not sure why this compiled in the first place)
- Remove unused local variables
- Avoid unintended fallthrough in switch statement
- Fix member variable initialization order in a constructor
- Avoid read of unintialized variable

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/11767 from Intel-tensorflow:yang/ci 0c2500b703b73bc340ccc9e3817c7ce54cc8d718
