Fix rsynaptic error on GPU

[dylanperdigao]

The RSynaptic has two errors, that can be solved by using a nn.Parameter() object.
Here are the errors that were solved with this pull request:

File "XXXX/.local/lib/python3.10/site-packages/snntorch/_neurons/rsynaptic.py", line 425, in _base_state_function
    base_fn_mem = self.beta.clamp(0, 1) * self.mem + base_fn_syn
 (Triggered internally at ../torch/csrc/autograd/python_anomaly_mode.cpp:114.)
  Variable._execution_engine.run_backward(  # Calls into the C++ engine to run the backward pass
one of the variables needed for gradient computation has been modified by an inplace operation: [torch.cuda.FloatTensor []] is at version 1; expected version 0 instead. Hint: the backtrace further above shows the operation that failed to compute its gradient. The variable in question was changed in there or anywhere later. Good luck!

File "XXXX/.local/lib/python3.10/site-packages/snntorch/_neurons/rsynaptic.py", line 538, in forward
    return x * self.V  # element-wise or global multiplication
 (Triggered internally at ../torch/csrc/autograd/python_anomaly_mode.cpp:114.)
  Variable._execution_engine.run_backward(  # Calls into the C++ engine to run the backward pass
one of the variables needed for gradient computation has been modified by an inplace operation: [torch.cuda.FloatTensor []] is at version 1; expected version 0 instead. Hint: the backtrace further above shows the operation that failed to compute its gradient. The variable in question was changed in there or anywhere later. Good luck!