Check for Inf and NaN within back!(::TrackedReal)

This is often checked for within user code, no reason to do that, let's
do it for them within `back!(::TrackedReal)`

src/optimise/train.jl

@@ -37,8 +37,6 @@ function train!(loss, data, opt; cb = () -> ())
   opt = runall(opt)
   @progress for d in data
     l = loss(d...)
-    isinf(l) && error("Loss is Inf")
-    isnan(l) && error("Loss is NaN")
     @interrupts back!(l)
     opt()
     cb() == :stop && break

src/tracker/scalar.jl

@@ -8,7 +8,15 @@ tracker(x::TrackedReal) = x.tracker
 
 track(f::Call, x::Real) = TrackedReal(Tracked(f, x, zero(x)))
 
-back!(x::TrackedReal) = back!(x, 1)
+function back!(x::TrackedReal)
+    if isinf(x)
+        error("Loss is Inf")
+    end
+    if isnan(x)
+        error("Loss is NaN")
+    end
+    return back!(x, 1)
+end
 
 function Base.show(io::IO, x::TrackedReal)
   show(io, data(x))