[BUG] dtype in _split_and_pad_sequence is established using incorrect dim

[KubaMichalczyk]

In torchrl/objectives/value/utils.py#L287-L290, the code currently uses tensor.shape[-2] to choose between torch.int16 and torch.int32. This should use the time dimension (time_dim) instead, especially since it's used in _fast_td_lambda_return_estimate where the inputs are transposed first. As a result, the condition may be incorrect for certain input shapes (when the size of time_dim exceeds the int16 range, but F dimension of _fast_td_lambda_return_estimate inputs is within this range).

Code Reference:

    # int16 supports length up to 32767
    dtype = (
        torch.int16 if tensor.shape[-2] < torch.iinfo(torch.int16).max else torch.int32
    )

Proposed Fix:

    dtype = (
        torch.int16 if tensor.size(time_dim) < torch.iinfo(torch.int16).max else torch.int32
    )

[vmoens]

You are correct! Do you want to push the fix or should I?