use_one_betas_per_video does not work when the frame size of provided initial betas is larger than one

[yl-1993]

When we have initial betas and the frame size is larger than 1, betas will keep its original shape according to this line, no matter whether use_one_betas_per_video is true or false.

In this way, if initial betas is (21, 10), then the fitted betas has 21 different values even when use_one_betas_per_video=True.
The expected behavior is when use_one_betas_per_video=True, the fitted betas across frames the same, with actual shape (1, 10).

For example,

if self.use_one_betas_per_video:
    if 'betas' not in init_dict:
        betas = torch.zeros(1, self.body_model.betas.shape[-1]).to(
                self.device)
    else:
        betas = init_dict['betas'].mean(dim=0, keepdim=True)
    ret_dict['betas'] = betas

[LazyBusyYang]

Fixed in PR99.