bug in tanhcnngaussianpolicy

[Yuichi-Nakababayashi]

TanhCNNGaussianPolicy class in gaussian_policy.py, following line may be incorrect.

self.last_fc_log_std = nn.Linear(last_hidden_size, action_dim)

as in case there're no hidden layers, last_hidden_size = self.input_width * self.input_width, wherase input dimension is not always the same as the dimension of CNN-output, which should be the same as the input dimension of the last layer.

Thought, I don't know a quick fix to that problem,,,