Giuseppe:

nn.GRU


somewhere in the code you are trying to matmul two tensors with shapes (16,5) and (15,32) but it is impossible, because to multiply two 2-dimensional tensors you need tensor1.shape[1] = tensor2.shape[0]
Looks like you are sending incorrect values to nn.GRU (Inputs: input, tensor containing the features of the input sequence, h_0, the initial hidden state for each element in the batch)