[Feature suggestion] Identifiers not on both sides of the expression

This seems like it should be (intuitively) plausible:

rearrange(x, 'b -> a b c', a=1, c=1)

to essentially push a vector to be compatible with some other tensors (for broadcasting operations). Currently this throws an error.

One (sort of ugly) workaround is:

rearrange(x, '(a b c) -> a b c', a=1, c=1)

However, it seems like this is a bit redundant and it obfuscates the intent a bit. Thoughts?

[arogozhnikov]

@jotaf98 it is done and available in master branch.

Axes of size 1 can be written as 1, not ().

But also: reduced axes can be anonymous

reduce(image, 'h w 3 -> h w', 'mean')

as well as repeated axes

repeat(image_bw, 'h w -> h w 3')

Same applies to subaxes - name is not required if those are reduced or repeated.

[arogozhnikov]

@serycjon use repeat if you need to introduce new named axes. Rearrange is strict towards missing identifiers on any side by design