[Enhancement] Improve ProfilerHook beaviour when using a relative (or empty) dir_name (#2119)

* Add default dir_name and make non-absolute dir_name relative to runner.work_dir

* Add log message and change default dir name

mmcv/runner/hooks/profiler.py

 # Copyright (c) OpenMMLab. All rights reserved.
+import os.path as osp
 import warnings
 from typing import Callable, List, Optional, Union
 
@@ -131,6 +132,15 @@ class ProfilerHook(Hook):
                     raise ImportError('please run "pip install '
                                       'torch-tb-profiler" to install '
                                       'torch_tb_profiler')
+                if 'dir_name' not in trace_cfg:
+                    trace_cfg['dir_name'] = osp.join(runner.work_dir,
+                                                     'tf_tracing_logs')
+                elif not osp.isabs(trace_cfg['dir_name']):
+                    trace_cfg['dir_name'] = osp.join(runner.work_dir,
+                                                     trace_cfg['dir_name'])
+                runner.logger.info(
+                    'tracing files of ProfilerHook will be saved to '
+                    f"{trace_cfg['dir_name']}.")
                 _on_trace_ready = torch.profiler.tensorboard_trace_handler(
                     **trace_cfg)
             else: