Add len to OrderedDict types (#21651)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21651
ghimport-source-id: 0bba5c1930865e2d18b18782ba8c8990b0761d4d
Differential Revision: D15767795
Pulled By: bwasti
fbshipit-source-id: 70e27176897b0f977c9034ffb3ad21091c91e12e
