set a reasonable default platform in each container

[ssube]

When the client does not request a specific execution provider, the server is selecting the first available one from the list, which may not be a good one. Use the default provider/platform from the params.json file.

2023-01-24T17:57:01.996197433Z 100.64.0.21 - - [24/Jan/2023 17:57:01] "POST /api/txt2img?cfg=6.00&steps=25&scheduler=euler-a&seed=-1&prompt=an+astronaut+eating+a+hamburger&negativePrompt=&model=&platform=&upscaling=&correction=&width=512&height=512 HTTP/1.1" 200 -
2023-01-24T17:57:03.026471064Z request from 100.64.0.21: 25 rounds of EulerAncestralDiscreteScheduler using /data/models/stable-diffusion-onnx-v1-5 on DmlExecutionProvider, 512x512, 6.0, 442657531 - an astronaut eating a hamburger

[ssube]

This is already the case, I had the server params set to platform.default: "amd", which attempted (and failed) to use the DmlExecutionProvider. The CUDA container should not have a params file with the default set to AMD.