Add symmteric accum method for empty gradients

[DhairyaLGandhi]

Continues from #1576

cc @oxinabox @CarloLucibello

PR Checklist

• Tests are added
• Documentation, if applicable

[oxinabox]

I was surprised this wasn't included before, but figured you had considered that

[DhairyaLGandhi]

This is the SciMLSensitivity/Core8 results with this branch

     Testing Running tests...
┌ Warning: Potential performance improvement omitted. EnzymeVJP tried and failed in the automated AD choice algorithm. To show the stack trace, set SciMLSensitivity.STACKTRACE_WITH_VJPWARN[] = true. To turn off this printing, add `verbose = false` to the `solve` call.
└ @ SciMLSensitivity ~/Downloads/arpa/jsmo/t2/SciMLSensitivity.jl/src/concrete_solve.jl:24

┌ Warning: Potential performance improvement omitted. ReverseDiffVJP tried and failed in the automated AD choice algorithm. To show the stack trace, set SciMLSensitivity.STACKTRACE_WITH_VJPWARN[] = true. To turn off this printing, add `verbose = false` to the `solve` call.
└ @ SciMLSensitivity ~/Downloads/arpa/jsmo/t2/SciMLSensitivity.jl/src/concrete_solve.jl:68

┌ Warning: The `alias_u0` keyword argument is deprecated. Please use a NonlinearAliasSpecifier, e.g. `alias = NonlinearAliasSpecifier(alias_u0 = true)`.
└ @ DiffEqBase ~/.julia/packages/DiffEqBase/5hvMq/src/solve.jl:1147
┌ Warning: The `alias_u0` keyword argument is deprecated. Please use a NonlinearAliasSpecifier, e.g. `alias = NonlinearAliasSpecifier(alias_u0 = true)`.
└ @ DiffEqBase ~/.julia/packages/DiffEqBase/5hvMq/src/solve.jl:1147
┌ Warning: The `alias_u0` keyword argument is deprecated. Please use a NonlinearAliasSpecifier, e.g. `alias = NonlinearAliasSpecifier(alias_u0 = true)`.
└ @ DiffEqBase ~/.julia/packages/DiffEqBase/5hvMq/src/solve.jl:1147
┌ Warning: The `alias_u0` keyword argument is deprecated. Please use a NonlinearAliasSpecifier, e.g. `alias = NonlinearAliasSpecifier(alias_u0 = true)`.
└ @ DiffEqBase ~/.julia/packages/DiffEqBase/5hvMq/src/solve.jl:1147
┌ Warning: `ignore(f)` is deprecated, use `ChainRulesCore.ignore_derivatives(f)` instead.
│   caller = adjoint at deprecated.jl:31 [inlined]
└ @ Core ~/Downloads/arpa/jsmo/t2/Zygote.jl/src/deprecated.jl:31
565.251994 seconds (375.47 M allocations: 23.189 GiB, 1.46% gc time, 36.08% compilation time: <1% of which was recompilation)
┌ Warning: Independent variable t should be defined with @independent_variables t.
└ @ ModelingToolkit ~/.julia/packages/ModelingToolkit/Z9mEq/src/utils.jl:119
┌ Warning: The `alias_u0` keyword argument is deprecated. Please use a NonlinearAliasSpecifier, e.g. `alias = NonlinearAliasSpecifier(alias_u0 = true)`.
└ @ DiffEqBase ~/.julia/packages/DiffEqBase/5hvMq/src/solve.jl:1147
 66.065494 seconds (114.91 M allocations: 7.082 GiB, 5.25% gc time, 77.43% compilation time: 2% of which was recompilation)
Test Summary:    | Pass  Total      Time
SciMLSensitivity |    7      7  10m31.3s
631.352301 seconds (490.40 M allocations: 30.272 GiB, 1.85% gc time, 40.41% compilation time: <1% of which was recompilation)
     Testing SciMLSensitivity tests passed 

[DhairyaLGandhi]

bump @CarloLucibello