IndexError: index 29 is out of bounds for dimension 0 with size 29

### Describe the bug

I have three problems because of the same reason.
1) TypeError: unsupported operand type(s) for +=: 'NoneType' and 'int'
# upon completion increase step index by one
self._step_index += 1 <---Error [here](https://github.com/huggingface/diffusers/blob/main/src/diffusers/schedulers/scheduling_flow_match_euler_discrete.py#L303)
2) IndexError: index 29 is out of bounds for dimension 0 with size 29
sigma_next = self.sigmas[self.step_index + 1] <--- Error [here](https://github.com/huggingface/diffusers/blob/main/src/diffusers/schedulers/scheduling_flow_match_euler_discrete.py#L295)
3) RuntimeError: Already borrowed
if _truncation is not None:
self._tokenizer.no_truncation() <--- Error here
Example: https://github.com/huggingface/tokenizers/issues/537
The reason, as I understood, is threads. Do you know, how can I solve this problem?

### Reproduction
```
from diffusers import (
FluxPipeline,
FlowMatchEulerDiscreteScheduler,
)
import torch

pipeline = FluxPipeline.from_pretrained(
"black-forest-labs/FLUX.1-schnell", torch_dtype=torch.bfloat16
).to("cuda")

seed = 42
height = 720
width = 1280

generator = torch.Generator(device="cuda").manual_seed(seed)

pipeline(
prompt=prompt + ", highly detailed, all is depicted as silhouettes, without words",
guidance_scale=0.,
# num_inference_steps=10,
height=height,
width=width,
generator=generator,
max_sequence_length=256,
).images[0]
```
### Logs

```shell
For example:
Traceback (most recent call last):
File "/opt/conda/lib/python3.10/site-packages/flask/app.py", line 1473, in wsgi_app
response = self.full_dispatch_request()
File "/opt/conda/lib/python3.10/site-packages/flask/app.py", line 882, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/opt/conda/lib/python3.10/site-packages/flask/app.py", line 880, in full_dispatch_request
rv = self.dispatch_request()
File "/opt/conda/lib/python3.10/site-packages/flask/app.py", line 865, in dispatch_request
return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args) # type: ignore[no-any-return]
File "/app/main.py", line 29, in generate_image
image = imagegen.run(**data)
File "/app/image_generator.py", line 102, in run
return generate_image()
File "/app/image_generator.py", line 89, in generate_image
return self.pipeline(
File "/opt/conda/lib/python3.10/site-packages/torch/utils/_contextlib.py", line 115, in decorate_context
return func(*args, **kwargs)
File "/opt/conda/lib/python3.10/site-packages/diffusers/pipelines/flux/pipeline_flux.py", line 734, in __call__
latents = self.scheduler.step(noise_pred, t, latents, return_dict=False)[0]
File "/opt/conda/lib/python3.10/site-packages/diffusers/schedulers/scheduling_flow_match_euler_discrete.py", line 295, in step
sigma_next = self.sigmas[self.step_index + 1]
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'
```

### System Info

- 🤗 Diffusers version: 0.31.0.dev0
- Platform: Linux-5.4.0-171-generic-x86_64-with-glibc2.35
- Running on Google Colab?: No
- Python version: 3.10.13
- PyTorch version (GPU?): 2.2.1 (True)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Huggingface_hub version: 0.24.6
- Transformers version: 4.44.2
- Accelerate version: 0.34.0
- PEFT version: 0.12.0
- Bitsandbytes version: not installed
- Safetensors version: 0.4.4
- xFormers version: not installed
- Accelerator: NVIDIA RTX A6000, 46068 MiB
- Using GPU in script?:
- Using distributed or parallel set-up in script?:

### Who can help?

@yiyixuxu @sayakpaul @DN6