gth, feature_dim).   class_labels (torch.Tensor, optional, defaults to None) —
Optional class labels for conditioning. Their embeddings will be summed with the timestep embeddings.
timestep_cond — (torch.Tensor, optional, defaults to None):
Conditional embeddings for timestep. If provided, the embeddings will be summed with the samples passed
through the self.time_embedding layer to obtain the timestep embeddings.   attention_mask (torch.Tensor