Fix Muon optimizer module path (deepspeedai#7802)

Update the Muon optimizer import to its current location under
deepspeed.runtime.zero.muon, so it’s correctly registered as a supported
ZeRO optimizer. This prevents the “untested optimizer” assertion from
firing.

Signed-off-by: Masahiro Tanaka <mtanaka@anyscale.com>

Author: tohtana

deepspeed/runtime/zero/utils.py

@@ -47,7 +47,7 @@ class ZeRORuntimeException(Exception):
 
 # Add MuonWithAuxAdam to supported list if muon is installed
 try:
-    from deepspeed.runtime.muon_optimizer import MuonWithAuxAdam
+    from deepspeed.runtime.zero.muon.muon_optimizer import MuonWithAuxAdam
     ZERO_SUPPORTED_OPTIMIZERS.append(MuonWithAuxAdam)
 except ImportError:
     pass