🙋Flexible Directory Structure Support for Custom Pipeline Scenarios

## 🚨 Flexible Directory Structure for Custom Pipeline Scenarios Raised Error!
When I have defined some custom_pipelines in local and the directory structure looks like:
```bash
custom_pipelines
├── custom_modules
│ ├── autoencoders
│ │ └── vae_a.py
│ └── transformers
│ ├── transformer_a.py
│ └── transformer_b.py
├── pipeline_a.py
└── pipeline_b.py
```

then I try to use `custom_pipeline` like:

```py
import torch
from diffusers import StableDiffusion3Pipeline

pipe = StableDiffusion3Pipeline.from_pretrained(
"stabilityai/stable-diffusion-3-medium-diffusers",
custom_pipeline="/path/to/my_custom_pipeline.py",
)
```

It raises error like:
```
> python ****/playground/test_pipes/custom_pipelines/test.py
Traceback (most recent call last):
File "****/playground/test_pipes/custom_pipelines/test.py", line 4, in
pipe = StableDiffusion3Pipeline.from_pretrained(
File "****/lib/python3.8/site-packages/huggingface_hub/utils/_validators.py", line 114, in _inner_fn
return fn(*args, **kwargs)
File "****/diffusers/src/diffusers/pipelines/pipeline_utils.py", line 699, in from_pretrained
cached_folder = cls.download(
File "****/lib/python3.8/site-packages/huggingface_hub/utils/_validators.py", line 114, in _inner_fn
return fn(*args, **kwargs)
File "****/diffusers/src/diffusers/pipelines/pipeline_utils.py", line 1402, in download
pipeline_class = _get_pipeline_class(
File "****/diffusers/src/diffusers/pipelines/pipeline_loading_utils.py", line 347, in _get_pipeline_class
return _get_custom_pipeline_class(
File "****/diffusers/src/diffusers/pipelines/pipeline_loading_utils.py", line 326, in _get_custom_pipeline_class
return get_class_from_dynamic_module(
File "****/lib/python3.8/site-packages/huggingface_hub/utils/_validators.py", line 114, in _inner_fn
return fn(*args, **kwargs)
File "****/diffusers/src/diffusers/utils/dynamic_modules_utils.py", line 447, in get_class_from_dynamic_module
final_module = get_cached_module_file(
File "****/lib/python3.8/site-packages/huggingface_hub/utils/_validators.py", line 114, in _inner_fn
return fn(*args, **kwargs)
File "****/diffusers/src/diffusers/utils/dynamic_modules_utils.py", line 336, in get_cached_module_file
shutil.copy(os.path.join(pretrained_model_name_or_path, module_needed), submodule_path / module_needed)
File "****/lib/python3.8/shutil.py", line 409, in copy
copyfile(src, dst, follow_symlinks=follow_symlinks)
File "****/lib/python3.8/shutil.py", line 259, in copyfile
with open(src, 'rb') as fsrc, open(dst, 'wb') as fdst:
FileNotFoundError: [Errno 2] No such file or directory: '****/playground/test_pipes/custom_pipelines/custom_modules.transformers.transformer_sd3.py'
```

## 🕵🏻‍♂️ Reason for Error

It raised error because all files involved in relative imports within `custom_pipelines.py` will be copied to to subfolder of `HF_MODULES_CACHE`, and the corresponding [code](https://github.com/huggingface/diffusers/blob/main/src/diffusers/utils/dynamic_modules_utils.py#L317-L336) for this copy logic is:
```py
# Check we have all the requirements in our environment
modules_needed = check_imports(resolved_module_file)

# Now we move the module inside our cached dynamic modules.
full_submodule = DIFFUSERS_DYNAMIC_MODULE_NAME + os.path.sep + submodule
create_dynamic_module(full_submodule)
submodule_path = Path(HF_MODULES_CACHE) / full_submodule
if submodule == "local" or submodule == "git":
# We always copy local files (we could hash the file to see if there was a change, and give them the name of
# that hash, to only copy when there is a modification but it seems overkill for now).
# The only reason we do the copy is to avoid putting too many folders in sys.path.
shutil.copyfile(resolved_module_file, submodule_path / module_file)
for module_needed in modules_needed:
if len(module_needed.split(".")) == 2:
module_needed = "/".join(module_needed.split("."))
module_folder = module_needed.split("/")[0]
if not os.path.exists(submodule_path / module_folder):
os.makedirs(submodule_path / module_folder)
module_needed = f"{module_needed}.py"
shutil.copyfile(os.path.join(pretrained_model_name_or_path, module_needed), submodule_path / module_needed)
```

In my case, my custom pipeline has relative imports like:
```py
from .custom_modules.transformers.transformer_sd3 import funcs_i_need
```
it will be added to `modules_needed` as `'custom_modules.transformers.transformer_sd3'`, and in loop `for module_needed in modules_needed`, it will miss the process in `if len(module_needed.split(".")) == 2` because there are 3! Then `shutil.copyfile` will try to copy `'****/custom_pipelines/custom_modules.transformers.transformer_sd3.py'` which is **NOT a path at all** and raise error!

## 👀 Might be a solution
Maybe we could do a minimal revision here to support more flexible directory structure here:
```diff
if submodule == "local" or submodule == "git":
# We always copy local files (we could hash the file to see if there was a change, and give them the name of
# that hash, to only copy when there is a modification but it seems overkill for now).
# The only reason we do the copy is to avoid putting too many folders in sys.path.
shutil.copyfile(resolved_module_file, submodule_path / module_file)
for module_needed in modules_needed:
- if len(module_needed.split(".")) == 2:
+ if len(module_needed.split(".")) >= 2:
module_needed = "/".join(module_needed.split("."))
- module_folder = module_needed.split("/")[0]
+ module_folder = module_needed.rsplit("/", 1)[0]
if not os.path.exists(submodule_path / module_folder):
os.makedirs(submodule_path / module_folder)
module_needed = f"{module_needed}.py"
shutil.copyfile(os.path.join(pretrained_model_name_or_path, module_needed), submodule_path / module_needed)
```

WDYT @yiyixuxu @sayakpaul @DN6 , happy to hear your suggestions☺️