support slicing on nn.Sequential

[silky]

it'd be great to pick out elements of of a sequential with array slicing:

s = nn.Sequential(...)
# Drop the last element
a = s[0:-1]

[acgtyrant]

I agree this idea!

[acgtyrant]

However, the official seems does not agree...

[soumith]

@colesbury this has come up again (last in #1114). Shall we reconsider?

[vishwakftw]

I attempted getting a slice to work. Is this fine?

def __getitem__(self, inp):
    if isinstance(inp, slice):
        ret_seq = Sequential()
        seq_list = list(self._modules.items())
        for i in range(*inp.indices(len(self._modules))):
            ret_seq.add_module(seq_list[i][0], seq_list[i][1])
        return ret_seq

If this is fine, I can issue a PR too. Let me know what you think. I believe this modification needs to be done in the Sequential class.

[vishwakftw]

cc: @soumith This issue has been open for quite sometime.