Manage context when opening a session against a micro target

Now that open() method in Session is gone with commit 42167f9
(Revert "[microTVM] Add open and close methods to Session class") use
contextlib to activate the context manager when opening a session
against a micro target.

python/tvm/driver/tvmc/runner.py

@@ -17,6 +17,7 @@
 """
 Provides support to run compiled networks both locally and remotely.
 """
+from contextlib import ExitStack
 import json
 import logging
 import pathlib
@@ -487,7 +488,10 @@ def run_module(
         # Remote RPC (running on a micro target)
         logger.debug("Running on remote RPC (micro target).")
         try:
-            session = tvm.micro.Session(project_.transport()).open()
+            with ExitStack() as stack:
+                session = tvm.micro.Session(project_.transport())
+            # Enter context manager 'session' so it's active from now on.
+            stack.enter_context(session)
         except:
             raise TVMCException("Could not open a session with the micro target.")
     else: