Fix access _output_size_init via self in FCRN.py

[l3str4nge]

Hello

In models/FCRN.py:

class fusion_ResNet(nn.Module):
    _output_size_init = (256, 256)

    def __init__(self, bs, layers, decoder, output_size=None, in_channels=3, pretrained=True, padding='ZeroPad'):

        ...
        if output_size == None:
            output_size = _output_size_init  # undefined variable, changed to self._output_size_init