fix lifetimes, add tensors iterator

[gvilums]

What does this PR do?

• Fixes the lifetimes of the TensorViews returned by the top-level SafeTensors object to depend on the underlying data instead of the SafeTensors object itself. This makes the API more flexible by allowing TensorViews to outlive the SafeTensors object that they were created from.
• Adds a method to SafeTensors to iterate over all contained TensorViews

[galqiwi]

I just barely created the same pr, and found out that this one already exists :)
The lifetime part is very crucial for me -- to the extent that I'm planning to use your branch in my pet project.
Maintainers, please review this PR

[Narsil]

Can you provide an actual example where this is impactful ?

I think adding a test showcasing the usefulness would help prevent future errors on this.

[gvilums]

@Narsil The basic pattern that this allows is the following:

        let serialized = b"...";

        let tensor = {
            let loaded = SafeTensors::deserialize(serialized).unwrap();
            loaded.tensor("test").unwrap()
        };
        
        do_something_with(tensor)

We want to get all the stored tensors without being tied to the lifetime of the SafeTensors object. This is of course a naive example - in our code we need this to be able to store our deserialized tensors without having to worry about keeping the SafeTensors object alive, which can get complicated because it itself borrows from the underlying data.

I added a test to check for this simple case.

[Narsil]

Thanks a lot for the tests/clarification. That makes it clearer and we can keep this for longer.

I'll release after this.