Solved by joekid in post #3 
              
              
                Thank you sir, I understood the item 1. 
Related 2, it is still unclear for me. I see that the batch should be (B, C, n_frames, H, W) with n_frames equals for all samples. 
By padding, do you say that a short sequence can be completed with frames of zeros until it achieves the desired n_frames equal…