Allow np.fill_diagonal to fill sub or super diagonals too

[ewmoore]

It would be really nice if np.fill_diagonal could fill other diagonals besides the main diagonal. This would match the offset argument of np.diagonal.

Shouldn't require much code to allow this.

[eric-wieser]

If we made diagonal return a writeable view, as we have long been planning to do, then you could spell that as:

np.diagonal(a, offset)[...] = values

[ewmoore]

Given that the docs currently say that 1.19 will return a read only view, I’d say our options are a little limited there. fill_diagonal can be changed to work now. There is no reason we can’t pursue both.

[eric-wieser]

Don't they say 1.9 not 1.19? If we made it writeable, that's 9 release cycles for people to fix their code, which might be enough.

[ewmoore]

I should read more carefully. It does say 1.9. Maybe now is the time then.

[charris]

I actually made that change some years ago, but it broke a (rather obscure) test. Maybe it is time to revisit it.

[rgommers]

@Aa88s please stop posting random comments. That's not acceptable on this repo nor any other open source project.