Various Chroma pipeline issues

### Describe the bug

Some minor issues that I have found while using the Chroma pipeline for the first time:

- if `sentencepiece` is not installed, you get a quite cryptic error message. Other pipelines such as Flux give you a nice error message that `sentencepiece` has to be installed
- `The config attributes {'guidance_embeds': False, 'pooled_projection_dim': 768} were passed to ChromaTransformer2DModel, but are not expected and will be ignored. Please verify your config.json configuration file.`
Chroma does not use CLIP, only T5, so there is no pooled projection. and does not use a guidance embedding as Flux - but I guess this is an issue with the config file, not the diffusers pipeline
- this code https://github.com/huggingface/diffusers/blob/03c3f69aa57a6cc2c995d41ea484195d719a240a/src/diffusers/pipelines/chroma/pipeline_chroma.py#L806 seems unnecessary, because dynamic timestep shifting is disabled in Chroma's scheduler config: https://huggingface.co/lodestones/Chroma1-HD/blob/main/scheduler/scheduler_config.json
confirmed by the actual timesteps that are used:
`(Pdb) timesteps
tensor([1000., 975., 950., 925., 900., 875., 850., 825., 800., 775.,
750., 725., 700., 675., 650., 625., 600., 575., 550., 525.,
500., 475., 450., 425., 400., 375., 350., 325., 300., 275.,
250., 225., 200., 175., 150., 125., 100., 75., 50., 25.],
device='cuda:0')`
- Chroma's official repo is `lodestones/Chroma1-HD` now. This could be updated in your pipeline example code: https://huggingface.co/docs/diffusers/main/api/pipelines/chroma

### Reproduction

run the sample code here https://huggingface.co/docs/diffusers/main/api/pipelines/chroma

### Logs

```shell

```

### System Info

diffusers HEAD, python 3.11.11

### Who can help?

_No response_

Start by running the Chroma sample from the pipeline documentation and inspect the referenced code in src/diffusers/pipelines/chroma/pipeline_chroma.py around line 806. Review each reported dependency, configuration-warning, scheduler, and example-model issue separately. Done means the listed problems are fixed or clearly confirmed as external configuration issues, with the Chroma example updated to the current repository.

Scritto dal modello di indicizzazione a partire dal testo della issue.