print out the optimizer

Summary:
Pull Request resolved: https://github.com/facebookresearch/d2go/pull/402

print out the optimizer for easier debug

Reviewed By: newstzpz

Differential Revision: D40701959

fbshipit-source-id: 7b610e8f5771409632ae056cb9d34138b331adbc

d2go/optimizer/build.py

@@ -328,6 +328,8 @@ def build_optimizer_mapper(cfg, model):
             ret += f"Param group {idx}: {_param_group_str(group)}\n"
         return ret
 
+    logger.info(f"Using optimizer:\n{optimizer}")
+
     logger.info(
         f"optimizer parameter groups:\n{_param_groups_str(optimizer.param_groups)}"
     )