Enable package import from torchbenchmark and torchbenchmark.models. (#2200)

Summary:
More intuitive module import when installed as a package.
This PR adds LazyImport to all torchbenchmark models so that they will be loaded only when being used.

Pull Request resolved: #2200

Test Plan:
Run the following commands:
```
python install.py
pip install -e .
$ python -c "import torchbenchmark.models; print(torchbenchmark.models.densenet121.Model)"
<class 'torchbenchmark.models.densenet121.Model'>
$ python -c "import torchbenchmark; print(torchbenchmark.models.densenet121.Model)"
<class 'torchbenchmark.models.densenet121.Model'>
```

Fixes #2193

Reviewed By: drisspg

Differential Revision: D55024068

Pulled By: xuzhao9

fbshipit-source-id: c9572308d202afaf8e528adb6585b5db80936cbd

Author: xuzhao9

torchbenchmark/models/__init__.py

@@ -1 +1,30 @@
+import os
+import sys
+import importlib
+import importlib.util
+from pathlib import Path
+
+def _list_models_without_import():
+    def _is_non_empty(dirpath):
+        init_file_path = dirpath.joinpath("__init__.py")
+        return init_file_path.exists() and init_file_path.stat().st_size > 0
+    current_dir = Path(__file__).parent
+    subdirs = [entry for entry in current_dir.iterdir() if entry.is_dir()]
+    non_empty_subdirs = list(map(lambda x: x.name, filter(_is_non_empty, subdirs)))
+    return non_empty_subdirs
+
+
+class LazyImport:
+    def __init__(self, module_name: str):
+        self.module_name = module_name
+        self._module = None
+
+    def __getattr__(self, attr: str):
+        if self._module is None:
+            self._module = importlib.import_module(self.module_name, package=__name__)
+        return getattr(self._module, attr)
+
+
+for _model_name in _list_models_without_import():
+    globals()[_model_name] = LazyImport(f".{_model_name}")