Fix: bug with clamp operation on Alpha neuron

[dylanperdigao]

There was a problem with the Alpha neuron; the clamp operation could not be performed on an nn.Parameter on GPU. To correct this, the clamp operation was performed before and is now working.

The error was:

    return Alpha(
  File "XXXX/.local/lib/python3.10/site-packages/snntorch/_neurons/alpha.py", line 119, in __init__
    self._alpha_register_buffer(alpha, learn_alpha)
  File "XXXX/.local/lib/python3.10/site-packages/snntorch/_neurons/alpha.py", line 252, in _alpha_register_buffer
    self.register_buffer("alpha", alpha)
  File "XXXX/.local/lib/python3.10/site-packages/torch/nn/modules/module.py", line 1715, in __setattr__
    raise TypeError(f"cannot assign '{torch.typename(value)}' as parameter '{name}' "
TypeError: cannot assign 'torch.FloatTensor' as parameter 'alpha' (torch.nn.Parameter or None expected)