You have declared the fully connected layer  as Linear(in_features=2048, out_features=1000, bias=True). Do note that the linear layer will now only take inputs of size (batch_size,2048). However, the size of your input is (8,73728). You have one of two options.

Modify the linear layer to take an input of 73728 - Linear(in_features=73728, out_features=1000, bias=True)
Modify the input ‘x’ to size (8,2048). This could be done by changing the previous layers to downsample the data. This is the recommended solution. In the previous solution, the parameter count of the fc layer would be substantially large.