Fixed params count and input size for multiple inputs

[ajaynarayanan]

I faced three issues in the case of multiple inputs for the model,

1. 'total_input_size' was incorrectly calculated by multiplying all the tuple values of 'input_size'. Suppose, model's forward function requires 2 params of say size [(3, 256, 128), (3, 256, 128)]. Total_input_size was calculated by multiplying all the values { 3x256x128x3x256x128 } instead of multiplying each tuples and adding them { 3x256x128 + 3x256x128 }. The issue was fixed by multiplying tuple and adding them.
2. The 'total_params' were incorrectly calculated for multiple inputs case, fixed it by calculating from the model directly.
3. 'summary[layer]["output_shape"]' needed to be flattened for computing 'total_output'. This was fixed by using 'funcy' package's flatten method.

[JayJJChen]

the Multiple Inputs demo error in read me is actually caused by this bug