use pytorch built-in SiLU function to save GPU memory usage

[pppoe]

• Replaced the custom implemented nonlinearity function with the PyTorch built-in function SiLU.
• The built-in function saves significant GPU memory when specifying a large output size.
• According to PyTorch documentation, the formulation/result is identical.

On my 12GB GPU card, the following command gives a GPU OOM error without this PR while runs through with it.

python3 scripts/txt2img.py --H 1024 --W 1024 --plms --ckpt /data/models/SD/v2-1_512-ema-pruned.ckpt --config configs/stable-diffusion/v2-inference.yaml  --device cuda --prompt "pytorch logo" --n_sample 1  --n_iter 1

Tested under the default environment requirements. SiLU function has been available in PyTorch after 1.7.0.