fix: use finite differences to test ground truth

Author: avik-pal

lib/LuxTestUtils/src/autodiff.jl

@@ -2,7 +2,7 @@ struct Constant{T}
     val::T
 end
 
-# Zygote.jl on CPU
+# FiniteDiff.jl on CPU
 function ground_truth_gradient(f, args...)
     cdev = cpu_device()
     f_cpu = try
@@ -12,7 +12,7 @@ function ground_truth_gradient(f, args...)
                 be fixed by defining overloads using ConstructionBase.jl" err
         f
     end
-    return gradient(f_cpu, AutoZygote(), map(cdev, args)...)
+    return gradient(f_cpu, AutoFiniteDiff(), map(cdev, args)...)
 end
 
 # Zygote.jl
@@ -80,13 +80,13 @@ end
 """
     test_gradients(f, args...; skip_backends=[], broken_backends=[], kwargs...)
 
-Test the gradients of `f` with respect to `args` using the specified backends.
+Test the gradients of `f` with respect to `args` using the specified backends. The ground
+truth gradients are computed using FiniteDiff.jl on CPU.
 
 | Backend        | ADType              | CPU | GPU | Notes             |
 |:-------------- |:------------------- |:--- |:--- |:----------------- |
 | Zygote.jl      | `AutoZygote()`      | ✔   | ✔   |                   |
 | ForwardDiff.jl | `AutoForwardDiff()` | ✔   | ✖   | `len ≤ 32`        |
-| FiniteDiff.jl  | `AutoFiniteDiff()`  | ✔   | ✖   | `len ≤ 32`        |
 | Enzyme.jl      | `AutoEnzyme()`      | ✔   | ✖   | Only Reverse Mode |
 
 ## Arguments
@@ -157,7 +157,6 @@ function test_gradients(
     push!(backends, AutoZygote())
     if !on_gpu
         total_length ≤ 32 && push!(backends, AutoForwardDiff())
-        total_length ≤ 32 && push!(backends, AutoFiniteDiff())
         # TODO: Move Enzyme out of here once it supports GPUs
         if enable_enzyme_reverse_mode || ENZYME_TESTING_ENABLED[]
             mode = if enzyme_set_runtime_activity