Bug in building DictModule with old version of PyTorch.

[jintuzhang]

When using old versions of PyTorch (e.g., 1.10), building a mlcolvars.data.DictModule may cause the following error:

raise ValueError("Sum of input lengths does not equal the length of the input dataset!").

And this is caused by the change of the torch.utils.data.random_split method:

random_split In PyTorch 2.1:

def random_split(dataset: Dataset[T], lengths: Sequence[Union[int, float]],
                 generator: Optional[Generator] = default_generator) -> List[Subset[T]]:
    r"""
    Randomly split a dataset into non-overlapping new datasets of given lengths.

    If a list of fractions that sum up to 1 is given,
    the lengths will be computed automatically as
    floor(frac * len(dataset)) for each fraction provided.

    After computing the lengths, if there are any remainders, 1 count will be
    distributed in round-robin fashion to the lengths
    until there are no remainders left.

    Optionally fix the generator for reproducible results, e.g.:

    Example:
        >>> # xdoctest: +SKIP
        >>> generator1 = torch.Generator().manual_seed(42)
        >>> generator2 = torch.Generator().manual_seed(42)
        >>> random_split(range(10), [3, 7], generator=generator1)
        >>> random_split(range(30), [0.3, 0.3, 0.4], generator=generator2)

    Args:
        dataset (Dataset): Dataset to be split
        lengths (sequence): lengths or fractions of splits to be produced
        generator (Generator): Generator used for the random permutation.
    """

random_split In PyTorch 1.10:

def random_split(dataset: Dataset[T], lengths: Sequence[int],
                 generator: Optional[Generator] = default_generator) -> List[Subset[T]]:
    r"""
    Randomly split a dataset into non-overlapping new datasets of given lengths.
    Optionally fix the generator for reproducible results, e.g.:

    >>> random_split(range(10), [3, 7], generator=torch.Generator().manual_seed(42))

    Args:
        dataset (Dataset): Dataset to be split
        lengths (sequence): lengths of splits to be produced
        generator (Generator): Generator used for the random permutation.
    """

This method is invoked by

mlcolvar/mlcolvar/data/datamodule.py

Line 211 in e356f24

def _split(self, dataset):

Apparently, the _split method passes dataset length fractions to the random_split method, but the old random_split method only accepts explicit dataset lengths as parameters. Thus, it may be reasonable to modify the code to pass actual data lengths.