[Modular] Add single file support to Modular

[DN6]

What does this PR do?

This PR

1. Adds support for single file loading in ComponentSpec

2. Renames the repo argument to pretrained_model_name_or_path. IMO this makes more sense if the same argument supports both filepath and repo_ids. Saw that there was a comment in there considering this chase, so I added it here. Alternatively, we can introduce a filename argument, but I this is cleaner.

Snippet to test

import torch
from diffusers.modular_pipelines.modular_pipeline_utils import ComponentSpec
from diffusers import FluxTransformer2DModel

component = ComponentSpec(
    "transformer",
    pretrained_model_name_or_path="https://huggingface.co/black-forest-labs/FLUX.1-dev/blob/main/flux1-dev.safetensors",
    type_hint=FluxTransformer2DModel
)
model = component.load(torch_dtype=torch.bfloat16)

Fixes # (issue)

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