Add usage example of custom gradients

docs/src/internals/tracker.md

@@ -100,16 +100,16 @@ minus(a, b) = a - b
 Firstly, we must tell the tracker system to stop when it sees a call to `minus`, and record it. We can do this using dispatch:
 
 ```julia
-using Flux.Tracker: TrackedReal, track, @grad
+using Flux.Tracker: TrackedArray, track, @grad
 
-minus(a::TrackedReal, b::TrackedReal) = Tracker.track(minus, a, b)
+minus(a::TrackedArray, b::TrackedArray) = track(minus, a, b)
 ```
 
 `track` takes care of building a new `Tracked` object and recording the operation on the tape. We just need to provide a gradient definition.
 
 ```julia
 @grad function minus(a, b)
-  return minus(a.data, b.data), Δ -> (Δ, -Δ)
+  return minus(data(a), data(b)), Δ -> (Δ, -Δ)
 end
 ```
 
@@ -121,6 +121,19 @@ Note that in the backpropagator we don't call `data(a)`; we *do* in fact want to
 @grad a * b = data(a)*data(b), Δ -> (Δ*b, a*Δ)
 ```
 
+We can then calculate the first derivative of `minus` as follows:
+
+```julia
+a = param([1,2,3])
+b = param([3,2,1])
+
+c = minus(a, b)  # [-2.0 (tracked), 0.0 (tracked), 2.0 (tracked)]
+
+Tracker.back!(c, 1)
+Tracker.grad(a)  # [1.00, 1.00, 1.00]
+Tracker.grad(b)  # [-1.00, -1.00, -1.00]
+```
+
 For multi-argument functions with custom gradients, you likely want to catch not just `minus(::TrackedArray, ::TrackedArray)` but also `minus(::Array, TrackedArray)` and so on. To do so, just define those extra signatures as needed:
 
 ```julia