[POC] Image generation multi-concurrency idea

[dkalinowski]

Increased number of infer requests for each model
Added a way to select infer request by request_idx
Added GenerationRequest class to manage assignment of request_idx

[likholat]

@dkalinowski, we've discussed this POC with @Wovchena and @ilya-lavrenov. It looks like the simplest and most transparent solution is to create new inference_requests using clone() method.

The clone() method should be implemented at the following levels:

1. Tasks: Text2ImagePipeline, Image2ImagePipeline, InpaintingPipeline
2. Pipelines: StableDiffusionPipeline, StableDiffusionXLPipeline, ...
3. Models: SD3Transformer2DModel, UNet2DConditionModel, AutoencoderKL, ...

Example: Text2ImagePipeline:

When calling Text2ImagePipeline::clone(), internally it will:

1. Call clone() on the corresponding diffusion pipeline (e.g. StableDiffusionPipeline::clone(...)), which:
   1. Reconstructs a new Scheduler from the config of the original
   2. Calls clone() for each model in the pipeline (e.g. CLIPTextModel::clone(...)), where a new inference_request is created internally

[dkalinowski]

@dkalinowski, we've discussed this POC with @Wovchena and @ilya-lavrenov. It looks like the simplest and most transparent solution is to create new inference_requests using clone() method.

The clone() method should be implemented at the following levels:

1. Tasks: Text2ImagePipeline, Image2ImagePipeline, InpaintingPipeline
2. Pipelines: StableDiffusionPipeline, StableDiffusionXLPipeline, ...
3. Models: SD3Transformer2DModel, UNet2DConditionModel, AutoencoderKL, ...

Example: Text2ImagePipeline:

When calling Text2ImagePipeline::clone(), internally it will:

1. Call clone() on the corresponding diffusion pipeline (e.g. StableDiffusionPipeline::clone(...)), which:

   1. Reconstructs a new Scheduler from the config of the original
   2. Calls clone() for each model in the pipeline (e.g. CLIPTextModel::clone(...)), where a new inference_request is created internally

Thank you for the review. Here is the corrected version with the clone idea: #2190

Could you review before I proceed with remaining pipeline types? @likholat