Lite version: use mmcv without installing pytorch as a requirement (#367)

* use mmcv without installing pytorch

* add new line

* revise utils.init by try-except-else block

* add new line to utils.init

* del whitespace in new line

* add flake8 ignore to utils.__init__

mmcv/utils/__init__.py

+# flake8: noqa
 # Copyright (c) Open-MMLab. All rights reserved.
 from .config import Config, ConfigDict, DictAction
-from .env import TORCH_VERSION
-from .logging import get_logger, print_log
 from .misc import (check_prerequisites, concat_list, is_list_of, is_seq_of,
                    is_str, is_tuple_of, iter_cast, list_cast,
                    requires_executable, requires_package, slice_list,
                    tuple_cast)
-from .parrots_wrapper import (CUDA_HOME, BuildExtension, CppExtension,
-                              CUDAExtension, DataLoader, PoolDataLoader,
-                              SyncBatchNorm, _AdaptiveAvgPoolNd,
-                              _AdaptiveMaxPoolNd, _AvgPoolNd, _BatchNorm,
-                              _ConvNd, _ConvTransposeMixin, _InstanceNorm,
-                              _MaxPoolNd, get_build_config)
 from .path import (check_file_exist, fopen, is_filepath, mkdir_or_exist,
                    scandir, symlink)
 from .progressbar import (ProgressBar, track_iter_progress,
                           track_parallel_progress, track_progress)
-from .registry import Registry, build_from_cfg
 from .timer import Timer, TimerError, check_time
 
-__all__ = [
-    'Config', 'ConfigDict', 'DictAction', 'get_logger', 'print_log', 'is_str',
-    'iter_cast', 'list_cast', 'tuple_cast', 'is_seq_of', 'is_list_of',
-    'is_tuple_of', 'slice_list', 'concat_list', 'check_prerequisites',
-    'requires_package', 'requires_executable', 'is_filepath', 'fopen',
-    'check_file_exist', 'mkdir_or_exist', 'symlink', 'scandir', 'ProgressBar',
-    'track_progress', 'track_iter_progress', 'track_parallel_progress',
-    'Registry', 'build_from_cfg', 'Timer', 'TimerError', 'check_time',
-    'CUDA_HOME', 'SyncBatchNorm', '_AdaptiveAvgPoolNd', '_AdaptiveMaxPoolNd',
-    '_AvgPoolNd', '_BatchNorm', '_ConvNd', '_ConvTransposeMixin',
-    '_InstanceNorm', '_MaxPoolNd', 'get_build_config', 'BuildExtension',
-    'CppExtension', 'CUDAExtension', 'DataLoader', 'PoolDataLoader',
-    'TORCH_VERSION'
-]
+try:
+    import torch
+except ImportError:
+    __all__ = [
+        'Config', 'ConfigDict', 'DictAction', 'is_str', 'iter_cast',
+        'list_cast', 'tuple_cast', 'is_seq_of', 'is_list_of', 'is_tuple_of',
+        'slice_list', 'concat_list', 'check_prerequisites', 'requires_package',
+        'requires_executable', 'is_filepath', 'fopen', 'check_file_exist',
+        'mkdir_or_exist', 'symlink', 'scandir', 'ProgressBar',
+        'track_progress', 'track_iter_progress', 'track_parallel_progress',
+        'Timer', 'TimerError', 'check_time'
+    ]
+else:
+    from .env import TORCH_VERSION
+    from .logging import get_logger, print_log
+    from .parrots_wrapper import (CUDA_HOME, BuildExtension, CppExtension,
+                                  CUDAExtension, DataLoader, PoolDataLoader,
+                                  SyncBatchNorm, _AdaptiveAvgPoolNd,
+                                  _AdaptiveMaxPoolNd, _AvgPoolNd, _BatchNorm,
+                                  _ConvNd, _ConvTransposeMixin, _InstanceNorm,
+                                  _MaxPoolNd, get_build_config)
+    from .registry import Registry, build_from_cfg
+    __all__ = [
+        'Config', 'ConfigDict', 'DictAction', 'get_logger', 'print_log',
+        'is_str', 'iter_cast', 'list_cast', 'tuple_cast', 'is_seq_of',
+        'is_list_of', 'is_tuple_of', 'slice_list', 'concat_list',
+        'check_prerequisites', 'requires_package', 'requires_executable',
+        'is_filepath', 'fopen', 'check_file_exist', 'mkdir_or_exist',
+        'symlink', 'scandir', 'ProgressBar', 'track_progress',
+        'track_iter_progress', 'track_parallel_progress', 'Registry',
+        'build_from_cfg', 'Timer', 'TimerError', 'check_time', 'CUDA_HOME',
+        'SyncBatchNorm', '_AdaptiveAvgPoolNd', '_AdaptiveMaxPoolNd',
+        '_AvgPoolNd', '_BatchNorm', '_ConvNd', '_ConvTransposeMixin',
+        '_InstanceNorm', '_MaxPoolNd', 'get_build_config', 'BuildExtension',
+        'CppExtension', 'CUDAExtension', 'DataLoader', 'PoolDataLoader',
+        'TORCH_VERSION'
+    ]