Remove required from --arch (facebookresearch#993)

Summary:
As per fairinternal/fairseq-py#983, we find that you cannot just modify the defaults of arch because it will still "require" you to input something on the command line. I'm not sure why this is required where task, criterion, etc (and actually, anything else) are not. Additionally, the [argparse docs](https://docs.python.org/3/library/argparse.html#required) claim that required=True is bad form so should be avoided.
Pull Request resolved: fairinternal/fairseq-py#993

Differential Revision: D19446805

Pulled By: myleott

fbshipit-source-id: 53221bb8bc1cea66197c5cee48a307b88d0d20b7

fairseq/options.py

@@ -581,7 +581,7 @@ def add_model_args(parser):
     # 2) --arch argument
     # 3) --encoder/decoder-* arguments (highest priority)
     from fairseq.models import ARCH_MODEL_REGISTRY
-    group.add_argument('--arch', '-a', default='fconv', metavar='ARCH', required=True,
+    group.add_argument('--arch', '-a', default='fconv', metavar='ARCH',
                        choices=ARCH_MODEL_REGISTRY.keys(),
                        help='Model Architecture')
     # fmt: on