Remove input check for vector of vectors

[grero]

Summary

This attempts to address #512, which prevents kernels operating on vectors of unequal length from working properly.

Proposed changes

• ...
  Remove the check on input dimensions when the inputs are vectors of vectors, since there is nothing wrong in principal with having kernels operate on these kinds of inputs.

What alternatives have you considered?

The most obvious alternative would be to implement my own type, wrapping AbstractVector{<:AbstractVector{<:Real}}, but since this change would potentially benefit others with similar needs, a fix in KernelFunctions itself seemed more appropriate.
Breaking changes

The main change is that inputs checks that are currently failing would pass. This is potentially breaking, as it is possible that downstream code could rely on these checks.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -14.31 ⚠️

Comparison is base (ef6d459) 94.16% compared to head (6b153d9) 79.85%.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #513       +/-   ##
===========================================
- Coverage   94.16%   79.85%   -14.31%     
===========================================
  Files          52       52               
  Lines        1387     1385        -2     
===========================================
- Hits         1306     1106      -200     
- Misses         81      279      +198     

Impacted Files	Coverage Δ
src/utils.jl	69.87% <ø> (-21.59%)	⬇️

... and 18 files with indirect coverage changes

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[willtebbutt]

Thanks for opening this PR.

I understand why you've taken this approach, but I don't believe it's the correct approach, as whether or not the input-dimension comparison should be performed is a function of the kernel, not the input type. For example, it (should be) perfectly fine to feed a Vector{Vector{Float64}} into an SEKernel, and in that case we would generally like to check that the input dimensions match.

To my mind, a better implementation would be to switch the check on / off depending upon which kernel is employed.

@theogf @devmotion @st-- do any of you have thoughts on this problem? I'm starting to wonder whether having this check is more hassle than it's worth, because the fix looks like it's going to be a bit tricky to implement.

[devmotion]

To my mind, a better implementation would be to switch the check on / off depending upon which kernel is employed.

Yes, I agree, that seems to be the best and most general approach. Maybe also dim is too specific and instead we should add a function such as are_valid_inputs(kernel, x, y)::Bool as the basis of valid_inputs?