Added --cpu flag to train using CPU only (#2755)

docs/Training-ML-Agents.md

   details.
 * `--debug`: Specify this option to enable debug-level logging for some parts of the code.
 * `--multi-gpu`: Setting this flag enables the use of multiple GPU's (if available) during training.
-
-
+* `--cpu`: Forces training using CPU only.
 
 ### Training Config File
 

ml-agents/mlagents/trainers/learn.py

     sampler_file_path: Optional[str]
     docker_target_name: Optional[str]
     env_args: Optional[List[str]]
+    cpu: bool
 
     @property
     def fast_simulation(self) -> bool:
         nargs=argparse.REMAINDER,
         help="Arguments passed to the Unity executable.",
     )
+    parser.add_argument(
+        "--cpu", default=False, action="store_true", help="Run with CPU only"
+    )
+
     args = parser.parse_args(argv)
     return CommandLineOptions.from_argparse(args)
 
 
     jobs = []
     run_seed = options.seed
+    if options.cpu:
+        os.environ["CUDA_VISIBLE_DEVICES"] = "-1"
+
     if options.num_runs == 1:
         if options.seed == -1:
             run_seed = np.random.randint(0, 10000)