make outputsize work with symbolically wrapped lux models

[SebastianM-C]

I'm trying to make the Lux model in MTKNN a parameter, but we need to call LuxCore.outputsize to get the size for symbolic registration. The issue is that in the registered function we get a SymbolicUtils.BasicSymbolic{<:LuxCore.AbstractLuxLayer}, but there are not methods that work with that in LuxCore since we need to hit a AbstractLuxLayer

LuxCore.outputsize(layer::AbstractLuxLayer, x, rng::AbstractRNG)

We can't register LuxCore.outputsize since that would give us a Num as a result of the size calculation. Is this a good solution?

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All modified and coverable lines are covered by tests ✅

Project coverage is 80.42%. Comparing base (9df148a) to head (7dfe7d9).
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@@             Coverage Diff             @@
##           master    #1508       +/-   ##
===========================================
+ Coverage    8.79%   80.42%   +71.63%     
===========================================
  Files          51       51               
  Lines        4924     5078      +154     
===========================================
+ Hits          433     4084     +3651     
+ Misses       4491      994     -3497     

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[SebastianM-C]

The test failure is because CellMLToolkit doesn't declare a compat on Symbolics (and is the same as master) 😅

[SebastianM-C]

@ChrisRackauckas is this good to go?