Support a matrix of indexes to implement usual embedding layer.

[avostryakov]

I've tried to implement embedding layer like in lasagne (https://github.com/Lasagne/Lasagne/blob/master/lasagne/layers/embedding.py) and I discovered that I can't do like this:

import numpy as np
import cgt

def main():
input_var = cgt.matrix('input', dtype=np.int64)
w_glove = cgt.shared(np.zeros((1000, 300), dtype=np.float32))
output = w_glove[input_var]
f = cgt.function([input_var], [output])

input = np.ones((3, 3), dtype=np.int32)
print f(input)

if name == 'main':
main()

w_glove[input_var] should return tensor3. Other words it replaces sequences of word's indexes in sequences of word's vectors. Usual NLP operation.

As result I have to replace output = w_glove[input_var] on:
output = w_glove[cgt.flatten(input_var)]
output = cgt.reshape(output, (3, 3, 300))

to flat indexes in indexes' list and reshape result to tensor3 after it. I believe that cgt can do it more effectively.

[avostryakov]

Moreover, I work on Mac OS X 64 bit. If I write above:
input_var = cgt.matrix('input', dtype=np.int32)

As result, I'll have a following error in this line: output = w_glove[cgt.flatten(input_var)]:

File "/Users/vostryakov/Downloads/cgt-master/cgt/core.py", line 1899, in typ_apply
assert input_types[1] == TensorType('i8', 1)
AssertionError

It looks like cgt.flatten(input_var) automatically cast int32 to int64 if operation system is 64 bit.