Simplify `backed` internal checking

[ilan-gold]

Please describe your wishes and possible alternatives to achieve the desired result.

Right now AnnData.isbacked (i.e., X is None and AnnData.filename is present) is used for checking for a variety of different behaviors/conditions aside from just getting X:

1. It prevents making a view-of-a-view when backed mode is present refactor: allow multi-indexed backed mode #2407
2. Custom repr
3. Preventing string->categorical conversion when AnnData.is_view this is meant to prevent accidentally reading a backed array into memory, I guess, since it will materialize X
4. AnnData.T although this is actually just broken in general - you can't tranpose something backed and will get an AttributeError about .T on h5py.Dataset - again ????? fix: disallow transpose for "raw" backed array objects #2399
5. AnnData.concatenate: chore!: remove AnnData.concatenate #2370
6. A bunch of stuff in Raw
7. AnnData.__{del,set}item__: fix!: remove __delitem__ and __setitem__ from the AnnData object #2367
8. Making a copy of AnnData fix: disallow copy for "backed" objects #2406

I think these cases can succinctly be broken down into two categories:

• Broken/unnecessary behavior I think 5 (because it is deprecated) and 2 fall into this category. To solve this I would propose just remove the behavior
• Behavior that doesn't work anyway on h5py.Dataset, BaseCompressedSparseDataset or zarr.Array i.e., things that are broken if isbacked is False but the underlying class on X or a different elem is the same or similar to if it were True. This is 1, 3, 4, 6, and 8. For this we probably need a AnnData.predicate function or similar that crawls the AnnData object and checks for a condition at each "leaf node" i.e., is it an instance of class that has a certain attribute (like .T in case 4 above or copy where making a copy of e.g., a zarr.Array probably doesn't make sense). Here we would probably refactor {read,write}_dispatched to rely on this behavior as well and just factor out the "what do you do at a node" part to either read/write or return a boolean

The goal of this is to simplify things to the point where isbacked either doesn't exist or runs on the proposed predicate function. This would make #2357 much simpler because we can just take actions depending on whether any of the objects are "backed"

[ilan-gold]

Closed by linked PRs