Start on batched SVD support

[kshyatt]

No description provided.

[lkdvos]

I haven't looked in too much detail at the actual implementation, but I think there's two more high-level comments/questions I have here:

I'm a bit surprised about actually trying to implement batched_f(::AbstractTensorMap), since that doesn't really feel like what is going on (it's a single tensor after all), and more importantly that is not super convenient to switch to from higher-level packages, since that doesn't involve switching algorithm and instead requires switching the function. I was somehow expecting that we would call f(::AbstractTensorMap), which would then automatically dispatch to batched_f on the blocks.

A second comment is that if I'm not mistaken there now is logic for deciding whether or not to batch in both MAK and also here, which is probably something to discuss: I would say we should either implement batched_f to handle how to decide grouping/padding/etc, or alternatively decide that here and pass on the 3D arrays, but in this latter case it seems like the batching logic shouldn't really be in MAK, (I'm also leaning slightly towards having that logic in MAK tbh).

[kshyatt]

Yeah I kind of did some necromancy on this after the MAK changes so I think this is helpful :). I was kind of torn about whether to "hide the ball" and just call batched_svd_compact! under the hood if the user specified svd_compact!, maybe that's ok to do though since it just falls back to the non batched version anyway...

[kshyatt]

Managed to offload nearly all the packing and decision making to MAK, the main thing is we need the PR over there to avoid trying to call something that doesn't (yet) exist for the LAPACK bindings.