make DepthAnythingV2 more reusable

[edgarriba]

This pull request includes changes to the candle-examples and candle-transformers projects to improve memory management and simplify the codebase. The most important changes involve the use of Arc for shared ownership and the removal of lifetime parameters in the DepthAnythingV2 and DPTHead structures.

Improvements to memory management:

• candle-examples/examples/depth_anything_v2/main.rs: Updated the DepthAnythingV2::new function call to use Arc for shared ownership of dinov2.
• candle-transformers/src/models/depth_anything_v2.rs: Added Arc to the DepthAnythingV2 struct to manage shared ownership of DinoVisionTransformer.

Codebase simplification:

• candle-transformers/src/models/depth_anything_v2.rs: Removed lifetime parameters from DPTHead and DepthAnythingV2 structs, and updated their implementations accordingly. [1] [2] [3]
• candle-transformers/src/models/depth_anything_v2.rs: Replaced references to configuration fields with direct field accesses in DPTHead. [1] [2]

These changes enhance the code's readability and maintainability while improving memory management through shared ownership.

[LaurentMazare]

Thanks!

[edgarriba]

@LaurentMazare afer this first contribution, I realized that you don't have any command to run the whole dev-test (clippy,fmt,etc).

A useful tool for that would be just -- https://github.com/casey/just