Merge #1221

1221: DataLoader with NamedTuple r=CarloLucibello a=cossio

Just a couple of small changes, so that `DataLoader` can be created with a `NamedTuple` of tensors instead of `Tuple`. This way the tensors can be referred to by name. For example

```
train_loader = DataLoader((images = Xtrain, labels = Ytrain), batchsize=16)
batch = first(train_loader)
y = model(batch.images)
logitcrossentropy(y, batch.labels)
```

If we only use tuples, then in datasets with multiple tensors one has to be careful about the order in which the tensors are fed into the `DataLoader` constructor and be consistent with this elsewhere. With `NamedTuples` one just have to be consistent about the names used, which I think is a minor improvement.

CC @CarloLucibello 

### PR Checklist

- [x] Tests are added
- [x] Entry in NEWS.md
- [x] Documentation, if applicable

I don't think this qualifies as an API change. It's just a minor feature addition. So final review probably not required.

- [ ] Final review from `@MikeInnes` or `@dhairyagandhi96` (for API changes).


Co-authored-by: cossio <j.cossio.diaz@gmail.com>
Co-authored-by: cossio <cossio@users.noreply.github.com>

NEWS.md

@@ -1,5 +1,6 @@
 # v0.11
 * Change to `DataLoader`'s constructor [https://github.com/FluxML/Flux.jl/pull/1152]
+* Use `DataLoader` with `NamedTuple`s, so that tensors can be accessed by name [https://github.com/FluxML/Flux.jl/pull/1221].
 * Error if Dense layers weights and biases are not arrays [https://github.com/FluxML/Flux.jl/pull/1218].
 
 # v0.10.5

src/data/dataloader.jl

@@ -16,7 +16,7 @@ end
 An object that iterates over mini-batches of `data`, each mini-batch containing `batchsize` observations
 (except possibly the last one). 
 
-Takes as input a data tensors or a tuple of one or more such tensors. 
+Takes as input a single data tensor, or a tuple (or a named tuple) of tensors.
 The last dimension in each tensor is considered to be the observation dimension.
 
 If `shuffle=true`, shuffles the observations each time iterations are re-started.
@@ -57,6 +57,13 @@ Usage example:
     # train for 10 epochs
     using IterTools: ncycle 
     Flux.train!(loss, ps, ncycle(train_loader, 10), opt)
+
+    # can use NamedTuple to name tensors
+    train_loader = DataLoader((images=Xtrain, labels=Ytrain), batchsize=2, shuffle=true)
+    for datum in train_loader
+        @assert size(datum.images) == (10, 2)
+        @assert size(datum.labels) == (2,)
+    end
 """
 function DataLoader(data; batchsize=1, shuffle=false, partial=true)
     batchsize > 0 || throw(ArgumentError("Need positive batchsize"))
@@ -88,19 +95,16 @@ end
 
 _nobs(data::AbstractArray) = size(data)[end]
 
-function _nobs(data::Tuple)
+function _nobs(data::Union{Tuple, NamedTuple})
     length(data) > 0 || throw(ArgumentError("Need at least one data input"))
     n = _nobs(data[1])
-    if !all(x -> _nobs(x) == n, data[2:end])
+    if !all(x -> _nobs(x) == n, Base.tail(data))
         throw(DimensionMismatch("All data should contain same number of observations"))
     end
     return n
 end
 
-function _getobs(data::AbstractArray{T,N}, i) where {T,N}
+_getobs(data::AbstractArray, i) = data[ntuple(i -> Colon(), Val(ndims(data) - 1))..., i]
-    getindex(data, ntuple(i->Colon(), N-1)..., i)
+_getobs(data::Union{Tuple, NamedTuple}, i) = map(Base.Fix2(_getobs, i), data)
-end
 
-_getobs(data::Tuple, i) = map(x -> _getobs(x, i), data)
+Base.eltype(::DataLoader{D}) where D = D
-
-Base.eltype(d::DataLoader{D}) where D = D

test/data.jl

@@ -3,6 +3,7 @@
     Y = [1:5;]
 
     d = DataLoader(X, batchsize=2)
+    @inferred first(d)
     batches = collect(d)
     @test eltype(batches) == eltype(d) == typeof(X)
     @test length(batches) == 3
@@ -11,6 +12,7 @@
     @test batches[3] == X[:,5:5]
 
     d = DataLoader(X, batchsize=2, partial=false)
+    @inferred first(d)
     batches = collect(d)
     @test eltype(batches) == eltype(d) == typeof(X)
     @test length(batches) == 2
@@ -18,6 +20,7 @@
     @test batches[2] == X[:,3:4]
 
     d = DataLoader((X,), batchsize=2, partial=false)
+    @inferred first(d)
     batches = collect(d)
     @test eltype(batches) == eltype(d) == Tuple{typeof(X)}
     @test length(batches) == 2
@@ -25,6 +28,7 @@
     @test batches[2] == (X[:,3:4],)
 
     d = DataLoader((X, Y), batchsize=2)
+    @inferred first(d)
     batches = collect(d)
     @test eltype(batches) == eltype(d) == Tuple{typeof(X), typeof(Y)}
     @test length(batches) == 3
@@ -38,6 +42,22 @@
     @test batches[3][1] == X[:,5:5]
     @test batches[3][2] == Y[5:5]
 
+    # test with NamedTuple
+    d = DataLoader((x=X, y=Y), batchsize=2)
+    @inferred first(d)
+    batches = collect(d)
+    @test eltype(batches) == eltype(d) == NamedTuple{(:x, :y), Tuple{typeof(X), typeof(Y)}}
+    @test length(batches) == 3
+    @test length(batches[1]) == 2
+    @test length(batches[2]) == 2
+    @test length(batches[3]) == 2
+    @test batches[1][1] == batches[1].x == X[:,1:2]
+    @test batches[1][2] == batches[1].y == Y[1:2]
+    @test batches[2][1] == batches[2].x == X[:,3:4]
+    @test batches[2][2] == batches[2].y == Y[3:4]
+    @test batches[3][1] == batches[3].x == X[:,5:5]
+    @test batches[3][2] == batches[3].y == Y[5:5]
+
     # test interaction with `train!`
     θ = ones(2)
     X = zeros(2, 10)