PERF Ravel each array only once in BaseMultilayerPerceptron._backprop

[alexhenrie]

This change decreases neural net training time by about 1%.

Test program:

from sklearn.datasets import load_digits
from sklearn.neural_network import MLPClassifier
from neurtu import delayed, Benchmark

digits = load_digits(return_X_y=True)
X = digits[0][:,:10]
y = digits[0][:,11]

clf = MLPClassifier(solver='lbfgs', alpha=1e-5,
                    hidden_layer_sizes=(5, 2), random_state=1, max_iter=1000)

train = delayed(clf).fit(X, y)
print(Benchmark(wall_time=True, cpu_time=True, repeat=10)(train))

Before:

      wall_time  cpu_time                                                                                                     
mean   2.060248  2.052360
max    2.069880  2.054955
std    0.004758  0.001587

After:

      wall_time  cpu_time                                                                                                     
mean   2.042922  2.038958
max    2.045680  2.053701
std    0.002014  0.005943

[rth]

Thanks @alexhenrie ! LGTM.

[alexhenrie]

Thank you!