Enable stride != 1 for SConv2dLSTM

[jeshraghian]

SConv2dLSTM currently concatenates the hidden cell with the input cell, applies a single nn.Conv2d function of channel length 4*out_channel
The output is then split into 4 chunks (along the channel dimension), which are fed to each of the 4 LSTM gates
This means HxW of the hidden cell must be equal to HxW of input

Stride != 1 causes HxW to no longer be equal.
To fix this, a stride argument can trigger the conv to take place before concatenation of the hidden state mem and input x.
Far less efficient than running all conv operations in one hit, though I don't yet see a way around this.