exit but not use all memory

[Jian Ma]

when run sample with Vulkan in archlinux(with 16G memory),
It panic when try to create_buffer, but from memory monitor, not all memory used.

when try to use OpenGL backend, can sample the image, but it's pixel all dark.

thread 'main' panicked at 'wgpu error: Validation Error

Caused by:
    In Device::create_buffer
      note: label = `Buffer Src`
    Not enough memory left

', /home/ensonmj/Codes/wgpu/wgpu/src/backend/direct.rs:3095:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[Gadersd]

burn-wpu is very memory inefficient at the moment. I suggest using the torch backend for now until burn-wpu is better optimized.