Refine NAS benchmark docs and examples (#2800)

* Refine NAS benchmark docs and examples

* Improve docs of NAS benchmark

docs/en_US/NAS/Benchmarks.md

-# NAS Benchmarks (experimental)
+# NAS Benchmarks
 
 ```eval_rst
 ..  toctree::
@@ -8,12 +8,13 @@
 ```
 
 ## Introduction
+
 To imporve the reproducibility of NAS algorithms as well as reducing computing resource requirements, researchers proposed a series of NAS benchmarks such as [NAS-Bench-101](https://arxiv.org/abs/1902.09635), [NAS-Bench-201](https://arxiv.org/abs/2001.00326), [NDS](https://arxiv.org/abs/1905.13214), etc. NNI provides a query interface for users to acquire these benchmarks. Within just a few lines of code, researcher are able to evaluate their NAS algorithms easily and fairly by utilizing these benchmarks.
 
 ## Prerequisites
 
-* Please prepare a folder to household all the benchmark databases. By default, it can be found at `${HOME}/.nni/nasbenchmark`. You can place it anywhere you like, and specify it in `NASBENCHMARK_DIR` before importing NNI.
-* Please install `peewee` via `pip install peewee`, which NNI uses to connect to database.
+* Please prepare a folder to household all the benchmark databases. By default, it can be found at `${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 install `peewee` via `pip3 install peewee`, which NNI uses to connect to database.
 
 ## Data Preparation
 

examples/nas/benchmarks/nasbench101.sh

@@ -15,5 +15,5 @@ fi
 echo "Generating database..."
 rm -f ${NASBENCHMARK_DIR}/nasbench101.db ${NASBENCHMARK_DIR}/nasbench101.db-journal
 mkdir -p ${NASBENCHMARK_DIR}
-python -m nni.nas.benchmarks.nasbench101.db_gen nasbench_full.tfrecord
+python3 -m nni.nas.benchmarks.nasbench101.db_gen nasbench_full.tfrecord
 rm -f nasbench_full.tfrecord

examples/nas/benchmarks/nasbench201.sh

@@ -15,5 +15,5 @@ fi
 echo "Generating database..."
 rm -f ${NASBENCHMARK_DIR}/nasbench201.db ${NASBENCHMARK_DIR}/nasbench201.db-journal
 mkdir -p ${NASBENCHMARK_DIR}
-python -m nni.nas.benchmarks.nasbench201.db_gen a.pth
+python3 -m nni.nas.benchmarks.nasbench201.db_gen a.pth
 rm -f a.pth

examples/nas/benchmarks/nds.sh

@@ -16,5 +16,5 @@ unzip data.zip
 echo "Generating database..."
 rm -f ${NASBENCHMARK_DIR}/nds.db ${NASBENCHMARK_DIR}/nds.db-journal
 mkdir -p ${NASBENCHMARK_DIR}
-python -m nni.nas.benchmarks.nds.db_gen nds_data
+python3 -m nni.nas.benchmarks.nds.db_gen nds_data
 rm -rf data.zip nds_data

src/sdk/pynni/nni/nas/benchmarks/nasbench101/query.py

@@ -15,7 +15,7 @@ def query_nb101_trial_stats(arch, num_epochs, isomorphism=True, reduction=None,
     arch : dict or None
         If a dict, it is in the format that is described in
         :class:`nni.nas.benchmark.nasbench101.Nb101TrialConfig`. Only trial stats
-        matched will be returned. If none, architecture will be a wildcard.
+        matched will be returned. If none, all architectures in the database will be matched.
     num_epochs : int or None
         If int, matching results will be returned. Otherwise a wildcard.
     isomorphism : boolean

src/sdk/pynni/nni/nas/benchmarks/nasbench201/query.py

@@ -14,7 +14,7 @@ def query_nb201_trial_stats(arch, num_epochs, dataset, reduction=None, include_i
     arch : dict or None
         If a dict, it is in the format that is described in
         :class:`nni.nas.benchmark.nasbench201.Nb201TrialConfig`. Only trial stats
-        matched will be returned. If none, architecture will be a wildcard.
+        matched will be returned. If none, all architectures in the database will be matched.
     num_epochs : int or None
         If int, matching results will be returned. Otherwise a wildcard.
     dataset : str or None