Flag derivative WRT map itself as not implemented in chain rule

[gaurav-arya]

There are definitely scenarios where there ought to be a gradient WRT some parameters of the linear map, so this PR marks its gradient as not implemented rather than as NoTangent() to try to avoid correctness issues. Here's an example:

function f(p)
           A = LinearMap([p])
           return sum(A * [p])
end
# function is a fancy p^2, so gradient should be 2p

Annoyingly, this doesn't actually fix anything yet, at least for Zygote, because Zygote treats NotImplemented's just like zeros (https://github.com/FluxML/Zygote.jl/issues/1204). But it still seems like the right thing to do.

[codecov[bot]]

Codecov Report

Base: 99.47% // Head: 99.47% // No change to project coverage :thumbsup:

Coverage data is based on head (466f71a) compared to base (50f5bf9). Patch coverage: 100.00% of modified lines in pull request are covered.

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[dkarrasch]

I'm not familiar with the chain rules stuff, but I trust that this is correct. Seems to be closer to the actual fact that the product in itself is differentiable w.r.t. map (in contrast to NoTangent()), but it's just not implemented.