backend usage docs

docs/src/apis/backends.md

@@ -1,3 +1,53 @@
-# Batching
+# Backends
 
-[WIP]
+## Basic Usage
+
+```julia
+model = Chain(Affine(10, 20), σ, Affine(20, 15), softmax)
+xs = rand(10)
+```
+
+Currently, Flux's pure-Julia backend has no optimisations. This means that calling
+
+```julia
+model(rand(10)) #> [0.0650, 0.0655, ...]
+```
+
+directly won't have great performance. In order to support a computationally intensive training process, we really on a backend like MXNet or TensorFlow.
+
+This is easy to do. Just call either `mxnet` or `tf` on a model to convert it to a model of that kind:
+
+```julia
+mxmodel = mxnet(model, (10, 1))
+mxmodel(xs) #> [0.0650, 0.0655, ...]
+# or
+tfmodel = tf(model)
+tfmodel(xs) #> [0.0650, 0.0655, ...]
+```
+
+These new models look and feel exactly like every other model in Flux, including returning the same result when you call them, and can be trained as usual using `Flux.train!()`. The difference is that the computation is being carried out by a backend, which will usually give a large speedup.
+
+## Native Integration
+
+Flux aims to provide high-level APIs that work well across backends, but in some cases you may want to take advantage of features specific to a given backend. In these cases it's easy to "drop down" and use the backend's API directly, where appropriate. For example:
+
+```julia
+using MXNet
+Flux.loadmx()
+
+mxmodel = mx.FeedForward(model)
+```
+
+This returns a standard `mx.FeedForward` instance, just like you might have created using MXNet's usual API. You can then use this with MXNet's data provider implementation, custom optimisers, or distributed training processes.
+
+Same goes for TensorFlow, where it's easy to create a `Tensor` object:
+
+```julia
+using TensorFlow
+Flux.loadtf()
+
+x  = placeholder(Float32)
+y = Tensor(model, x)
+```
+
+This makes makes it easy to take advantage of Flux's model description and debugging tools while also getting the benefit of the work put into these backends. You can check out how this looks with the integration examples [here](https://github.com/MikeInnes/Flux.jl/tree/master/examples).

examples/integration-mx.jl

@@ -2,21 +2,6 @@ using Flux, MXNet
 
 Flux.loadmx()
 
-# Flux aims to provide high-level APIs that work well across backends, but in
-# some cases you may want to take advantage of features specific to a given
-# backend (or alternatively, Flux may simply not have an implementation of that
-# feature yet). In these cases it's easy to "drop down" and use the backend's
-# API directly, where appropriate.
-
-# In this example, both things are happening; firstly, Flux doesn't yet support
-# ConvNets in the pure-Julia backend, but this is invisible thanks to the use of
-# a simple "shim" type, `Conv`. This is provided by the library but could easily
-# have been user-defined.
-
-# Secondly, we want to take advantage of MXNet.jl's training process and
-# optimisers. We can simply call `mx.FeedForward` exactly as we would on a
-# regular MXNet model, and the rest of the process is trivial.
-
 conv1 = Chain(
   Input(28,28),
   Conv2D((5,5), out = 20), tanh,

examples/integration-tf.jl

@@ -1,21 +1,5 @@
 using Flux, Juno
 
-# Flux aims to provide high-level APIs that work well across backends, but in
-# some cases you may want to take advantage of features specific to a given
-# backend (or Flux may simply not have an implementation of that feature yet).
-# In these cases it's easy to "drop down" and use the backend's API directly,
-# where appropriate.
-
-# In this example, both things are happening; firstly, Flux doesn't yet support
-# ConvNets in the pure-Julia backend, but this is invisible thanks to the use of
-# a simple "shim" type, `Conv2D`. This is provided by the library but could easily
-# have been user-defined.
-
-# Secondly, we want to take advantage of TensorFlow.jl's training process and
-# optimisers. We can simply call `Tensor` exactly as we would on a regular
-# TensorFlow model, and the rest of the process trivially follows
-# TensorFlow.jl's usual API.
-
 conv1 = Chain(
   Reshape(28,28,1),
   Conv2D((5,5), out = 20), tanh,