[Feature suggestion] Support composition/decomposition of axes in einsum

[f-dangel]

Hi,

thanks for this great package! This is primarily a clarification question.

I ran into this comment in einops.einsum which states that the index un-grouping syntax of rearrange (e.g. '(a b c) -> (a b) c') is currently not supported in einsum. I wanted to know if there is a design problem that prevents supporting this, or whether it is simply not implemented. A first naive version would be to wrap the existing einsum inside two rearranges, but there are potential optimizations, e.g. merging parallel legs.

If you believe this can be a meaningful extension, I'd try to motivate it with some use cases.

Best,
Felix

[arogozhnikov]

Hi Felix,

I wanted to know if there is a design problem that prevents supporting this, or whether it is simply not implemented.

somewhat in the middle.

There are many potential wishes to einops.einsum, which require some investment (e.g. collapsed ellipsis, 1-axes, composition/decomposition of axes).

E.g. if you think of pattern

(a b c), (b c ...), ..., b ..., (a d) -> a b c d

You'll see this requires quite a number of steps to infer all shapes.

Alternative is to ask user to provide sufficient number of dimensions, e.g. in this case 'a', 'b' and 'c'

So I'll say there should be a strong reason to justify maintenance of this logic.

but there are potential optimizations, e.g. merging parallel legs

I expect backend framework to take care of that. Doing this on einops side may induce additional copies (frameworks can make these allocations transient/partial or even avoid them at all).

[f-dangel]

Thanks for the clarifications,

I understand it would require additional logic to infer dimensions, or cause more work for a user to specify them. For me, this feature is interesting because I'm working with operations that can be expressed neatly in this syntax and will allow me to replace rearrange -> einsum -> rearrange with a single call to einsum.

Feel free to close the issue.

Best,
Felix

I'm leaving behind my own cooked up version that supports the syntax. Maybe it's useful for someone else:

"""Einsum with support for ``einops``' index un-grouping syntax."""

from typing import Union

from einops import einsum as einops_einsum
from einops import rearrange
from einops.einops import Tensor
from torch import allclose, manual_seed, rand


def einsum(*tensors_and_pattern: Union[Tensor, str], **axes_lengths: int) -> Tensor:
    """Same as ``einops.einsum`` but supports index un-grouping notation.

    For example, the following operation does not work (yet) in ``einops.einsum``
    (https://github.com/arogozhnikov/einops/blob/fcd36c9b017d52e452a301e75c1373b75ec23ee0/einops/einops.py#L833-L834),
    but works with this version: ``einsum(A, B, '(a b) c, a b c -> (a b) c', a=a, b=b)``.

    Args:
        tensors_and_pattern:
            tensors: tensors of any supported library (numpy, tensorflow, pytorch, jax).
            pattern: string, einsum pattern, with commas separating specifications for
                each tensor. Pattern should be provided after all tensors.
        axes_lengths: Length of axes that cannot be inferred.

    Returns:
        Tensor of the same type as input, after processing with einsum.

    Raises:
        NotImplementedError: If the pattern contains unsupported features.
    """
    try:
        return einops_einsum(*tensors_and_pattern)
    except NotImplementedError as e:
        tensors, pattern = tensors_and_pattern[:-1], tensors_and_pattern[-1]
        if "(" not in pattern or ")" not in pattern:
            raise NotImplementedError from e

        # un-group the operands
        lefts, right = pattern.split("->")
        lefts = lefts.split(",")
        lefts_ungrouped = [l.replace("(", "").replace(")", "") for l in lefts]
        tensors_ungrouped = [
            rearrange(t, " -> ".join([l, l_u]), **axes_lengths) if l != l_u else t
            for t, l, l_u in zip(tensors, lefts, lefts_ungrouped)
        ]

        # compute the result with un-grouped indices
        right_ungrouped = right.replace("(", "").replace(")", "")
        pattern_ungrouped = " -> ".join([",".join(lefts_ungrouped), right_ungrouped])
        result_ungrouped = einops_einsum(*tensors_ungrouped, pattern_ungrouped)

        # group the indices in the result tensor
        return (
            rearrange(
                result_ungrouped, " -> ".join([right_ungrouped, right]), **axes_lengths
            )
            if right_ungrouped != right
            else result_ungrouped
        )


def test_einsum():
    """Test einsum with support for index un-grouping syntax."""
    manual_seed(0)

    a, b, c = (3, 4, 5)
    A = rand(a, b, c)
    B = rand(a * b, c)

    # NOTE Need to specify dims ``a, b`` although they could be inferred
    axes_lengths = dict(a=a, b=b)

    # no rearrange of result tensor
    C = einsum(A, B, "a b c, (a b) c -> a b c", **axes_lengths)
    C_truth = einsum(A, B.reshape(a, b, c), "a b c, a b c -> a b c")
    assert allclose(C, C_truth)

    # rearrange required before returning the result
    C = einsum(A, B, "a b c, (a b) c -> (a b) c", **axes_lengths)
    C_truth = einsum(A, B.reshape(a, b, c), "a b c, a b c -> a b c").reshape(a * b, c)
    assert allclose(C, C_truth)


if __name__ == "__main__":
    test_einsum()