PR #11721: [XLA:CPU] Add MpiCollectives to the the .pyi stubs

Imported from GitHub PR openxla/xla#11721

we forgot this in openxla/xla#7849.
Copybara import of the project:

--
3924cc0fbbb63e9503f38a59aede3b8e817b17fa by Clemens Giuliani <clemens@inailuig.it>:

[XLA:CPU] add missing type annotations for the mpi collectives

Merging this change closes #11721

FUTURE_COPYBARA_INTEGRATE_REVIEW=openxla/xla#11721 from inailuig:mpicollectives_pytype 3924cc0fbbb63e9503f38a59aede3b8e817b17fa
PiperOrigin-RevId: 627055918

Author: inailuig

third_party/xla/xla/python/xla_extension/__init__.pyi

@@ -549,6 +549,12 @@ def make_gloo_tcp_collectives(
     interface: Optional[str] = ...,
 ) -> CpuCollectives: ...
 
+class MpiCollectives(CpuCollectives):
+  def Init(self): ...
+  def Finalize(self): ...
+
+def make_mpi_collectives() -> MpiCollectives: ...
+
 def get_tfrt_cpu_client(
     asynchronous: bool = ...,
     distributed_client: Optional[DistributedRuntimeClient] = ...,