* Add `ROCSHMEM_CTX_INVALID` for invalid context handling
- Define `ROCSHMEM_CTX_INVALID` as {nullptr, nullptr}
- Add == and != operators to rocshmem_ctx_t
- Use `ROCSHMEM_CTX_INVALID` on failed context creation
- Skip ctx destroy if context is invalid
* Update docs for context create and destroy APIs usage and behavior
Avinash Kethineedi
955c22aeed