Validate provided arguments and act on --help.

def validate_args(args):
    """Validate provided arguments and act on --help."""
    if args.list_embedding_sources:
        print('Listing all sources for {} embeddings.'.format(
            args.embedding_name))
        print('Specify --embedding-name if you wish to '
              'list sources of other embeddings')
        print('')
        if args.embedding_name not in nlp.embedding.list_sources().keys():
            print('Invalid embedding name.')
            print('Only {} are supported.'.format(', '.join(
                nlp.embedding.list_sources().keys())))
            sys.exit(1)
        print(' '.join(nlp.embedding.list_sources()[args.embedding_name]))
        sys.exit(0)

    if not (args.embedding_path or args.embedding_name):
        print('You must specify either --embedding-path or --embedding-name ')
        print('Use --embedding-path to load and evaluate '
              'word embeddings from a Word2Vec text format '
              'or fastText binary format file')
        print('Use --embedding-name or to download one of '
              'the pre-trained embedding files included in GluonNLP.')
        sys.exit(1)

    if args.embedding_name and not args.embedding_source:
        print('Please also specify --embedding-source'
              ' to select the version of the pre-trained embedding. '
              'Use --list-embedding-sources to see all available sources')
        sys.exit(1)

    print(args)