split out template docs

This commit is contained in:
Mike J Innes 2017-02-02 12:34:36 +05:30
parent f3a9934858
commit 4351622f63
2 changed files with 97 additions and 98 deletions

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@ -67,101 +67,3 @@ We noted above that a "model" is a function with some number of trainable parame
foo = Chain(exp, sum, log)
foo([1,2,3]) == 3.408 == log(sum(exp([1,2,3])))
```
## The Template
*... Calculating Tax Expenses ...*
So how does the `Affine` template work? We don't want to duplicate the code above whenever we need more than one affine layer:
```julia
W₁, b₁ = randn(...)
affine₁(x) = W₁*x + b₁
W₂, b₂ = randn(...)
affine₂(x) = W₂*x + b₂
model = Chain(affine₁, affine₂)
```
Here's one way we could solve this: just keep the parameters in a Julia type, and define how that type acts as a function:
```julia
type MyAffine
W
b
end
# Use the `MyAffine` layer as a model
(l::MyAffine)(x) = l.W * x + l.b
# Convenience constructor
MyAffine(in::Integer, out::Integer) =
MyAffine(randn(out, in), randn(out))
model = Chain(MyAffine(5, 5), MyAffine(5, 5))
model(x1) # [-1.54458,0.492025,0.88687,1.93834,-4.70062]
```
This is much better: we can now make as many affine layers as we want. This is a very common pattern, so to make it more convenient we can use the `@net` macro:
```julia
@net type MyAffine
W
b
x -> W * x + b
end
```
The function provided, `x -> W * x + b`, will be used when `MyAffine` is used as a model; it's just a shorter way of defining the `(::MyAffine)(x)` method above.
However, `@net` does not simply save us some keystrokes; it's the secret sauce that makes everything else in Flux go. For example, it analyses the code for the forward function so that it can differentiate it or convert it to a TensorFlow graph.
The above code is almost exactly how `Affine` is defined in Flux itself! There's no difference between "library-level" and "user-level" models, so making your code reusable doesn't involve a lot of extra complexity. Moreover, much more complex models than `Affine` are equally simple to define.
### Sub-Templates
`@net` models can contain sub-models as well as just array parameters:
```julia
@net type TLP
first
second
function (x)
l1 = σ(first(x))
l2 = softmax(second(l1))
end
end
```
Just as above, this is roughly equivalent to writing:
```julia
type TLP
first
second
end
function (self::TLP)(x)
l1 = σ(self.first(x))
l2 = softmax(self.second(l1))
end
```
Clearly, the `first` and `second` parameters are not arrays here, but should be models themselves, and produce a result when called with an input array `x`. The `Affine` layer fits the bill so we can instantiate `TLP` with two of them:
```julia
model = TLP(Affine(10, 20),
Affine(20, 15))
x1 = rand(20)
model(x1) # [0.057852,0.0409741,0.0609625,0.0575354 ...
```
You may recognise this as being equivalent to
```julia
Chain(
Affine(10, 20), σ
Affine(20, 15), softmax)
```
given that it's just a sequence of calls. For simple networks `Chain` is completely fine, although the `@net` version is more powerful as we can (for example) reuse the output `l1` more than once.

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@ -0,0 +1,97 @@
# Model Templates
*... Calculating Tax Expenses ...*
So how does the `Affine` template work? We don't want to duplicate the code above whenever we need more than one affine layer:
```julia
W₁, b₁ = randn(...)
affine₁(x) = W₁*x + b₁
W₂, b₂ = randn(...)
affine₂(x) = W₂*x + b₂
model = Chain(affine₁, affine₂)
```
Here's one way we could solve this: just keep the parameters in a Julia type, and define how that type acts as a function:
```julia
type MyAffine
W
b
end
# Use the `MyAffine` layer as a model
(l::MyAffine)(x) = l.W * x + l.b
# Convenience constructor
MyAffine(in::Integer, out::Integer) =
MyAffine(randn(out, in), randn(out))
model = Chain(MyAffine(5, 5), MyAffine(5, 5))
model(x1) # [-1.54458,0.492025,0.88687,1.93834,-4.70062]
```
This is much better: we can now make as many affine layers as we want. This is a very common pattern, so to make it more convenient we can use the `@net` macro:
```julia
@net type MyAffine
W
b
x -> W * x + b
end
```
The function provided, `x -> W * x + b`, will be used when `MyAffine` is used as a model; it's just a shorter way of defining the `(::MyAffine)(x)` method above.
However, `@net` does not simply save us some keystrokes; it's the secret sauce that makes everything else in Flux go. For example, it analyses the code for the forward function so that it can differentiate it or convert it to a TensorFlow graph.
The above code is almost exactly how `Affine` is defined in Flux itself! There's no difference between "library-level" and "user-level" models, so making your code reusable doesn't involve a lot of extra complexity. Moreover, much more complex models than `Affine` are equally simple to define.
## Sub-Templates
`@net` models can contain sub-models as well as just array parameters:
```julia
@net type TLP
first
second
function (x)
l1 = σ(first(x))
l2 = softmax(second(l1))
end
end
```
Just as above, this is roughly equivalent to writing:
```julia
type TLP
first
second
end
function (self::TLP)(x)
l1 = σ(self.first(x))
l2 = softmax(self.second(l1))
end
```
Clearly, the `first` and `second` parameters are not arrays here, but should be models themselves, and produce a result when called with an input array `x`. The `Affine` layer fits the bill so we can instantiate `TLP` with two of them:
```julia
model = TLP(Affine(10, 20),
Affine(20, 15))
x1 = rand(20)
model(x1) # [0.057852,0.0409741,0.0609625,0.0575354 ...
```
You may recognise this as being equivalent to
```julia
Chain(
Affine(10, 20), σ
Affine(20, 15), softmax)
```
given that it's just a sequence of calls. For simple networks `Chain` is completely fine, although the `@net` version is more powerful as we can (for example) reuse the output `l1` more than once.