NAS benchmark docs update and backward-compat (#4256)

docs/en_US/NAS/Benchmarks.rst

@@ -15,14 +15,21 @@ Prerequisites
 -------------
 
 
-* Please prepare a folder to household all the benchmark databases. By default, it can be found at ``${HOME}/.nni/nasbenchmark``. You can place it anywhere you like, and specify it in ``NASBENCHMARK_DIR`` via ``export NASBENCHMARK_DIR=/path/to/your/nasbenchmark`` before importing NNI.
+* Please prepare a folder to household all the benchmark databases. By default, it can be found at ``${HOME}/.cache/nni/nasbenchmark``. Or you can place it anywhere you like, and specify it in ``NASBENCHMARK_DIR`` via ``export NASBENCHMARK_DIR=/path/to/your/nasbenchmark`` before importing NNI.
 * Please install ``peewee`` via ``pip3 install peewee``\ , which NNI uses to connect to database.
 
 Data Preparation
 ----------------
 
-To avoid storage and legality issues, we do not provide any prepared databases. Please follow the following steps.
+Option 1 (Recommended)
+^^^^^^^^^^^^^^^^^^^^^^
 
+You can download the preprocessed benchmark files via ``python -m nni.nas.benchmarks.download <benchmark_name>``, where ``<benchmark_name>`` can be ``nasbench101``, ``nasbench201``, and etc. Add ``--help`` to the command for supported command line arguments.
+
+Option 2
+^^^^^^^^
+
+.. note:: If you have files that are processed before v2.5, it is recommended that you delete them and try option 1.
 
 #. 
    Clone NNI to your machine and enter ``examples/nas/benchmarks`` directory.

nni/nas/benchmarks/constants.py

 import os
 
 
+ENV_NASBENCHMARK_DIR = 'NASBENCHMARK_DIR'
 ENV_NNI_HOME = 'NNI_HOME'
 ENV_XDG_CACHE_HOME = 'XDG_CACHE_HOME'
 DEFAULT_CACHE_DIR = '~/.cache'
@@ -10,7 +11,7 @@ def _get_nasbenchmark_dir():
     nni_home = os.path.expanduser(
         os.getenv(ENV_NNI_HOME,
                   os.path.join(os.getenv(ENV_XDG_CACHE_HOME, DEFAULT_CACHE_DIR), 'nni')))
-    return os.path.join(nni_home, 'nasbenchmark')
+    return os.getenv(ENV_NASBENCHMARK_DIR, os.path.join(nni_home, 'nasbenchmark'))
 
 
 DATABASE_DIR = _get_nasbenchmark_dir()

nni/nas/benchmarks/download.py

+import argparse
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser('NAS benchmark downloader')
+    parser.add_argument('benchmark_name', choices=['nasbench101', 'nasbench201', 'nds'])
+
+    args = parser.parse_args()
+
+    from .utils import download_benchmark
+    download_benchmark(args.benchmark_name)