Missing _calc_feat_flatten_mean_std in function.py

[nomadshark]

very good work.
there's a minor error near line 41 to 47, _calc_feat_flatten_mean_std is missed.
source_f, source_f_mean, source_f_std = _calc_feat_flatten_mean_std(source)
source_f_norm = (source_f - source_f_mean.expand_as(
source_f)) / source_f_std.expand_as(source_f)
source_f_cov_eye =
torch.mm(source_f_norm, source_f_norm.t()) + torch.eye(3)

target_f, target_f_mean, target_f_std = _calc_feat_flatten_mean_std(target)

[JarrentWu1031]

Thank you for your reminding! The missing function is added now.