[RPC] Lazily import micro when starting an RPC server (apache#6505)

* [RPC] Lazily import micro when starting an RPC server

Since apache#6334 the RPC server cannot be started unless USE_MICRO is enabled. I've tracked this down to an import in `python/tvn/exec/rpc_server.py`: `from tvm import micro` in the top level list of imports. This will mean that we try to import micro when it's not been built. Fix this by lazily importing micro when initializing an rpc server with micro enabled.

Change-Id: I8f22d81e215cfe4ac0662b0a99bdf02a3e91f90c

* fix lint

Change-Id: I8b78b678374bc82b3b66a7b3595ed4f1684e7d90

python/tvm/exec/rpc_server.py

@@ -25,7 +25,6 @@
 import sys
 import logging
 import tvm
-from tvm import micro
 from .. import rpc
 
 
@@ -70,6 +69,8 @@ def init_utvm(args):
     args : argparse.Namespace
         parsed args from command-line invocation
     """
+    from tvm import micro  # pylint: disable=import-outside-toplevel
+
     if args.utvm_dev_config and args.utvm_dev_id:
         raise RuntimeError("only one of --utvm-dev-config and --utvm-dev-id allowed")