refactor: simplify LeJEPA loss and module surface (#1926)

* refactor: update LeJEPA modules

* fix: align LeJEPA tests and defaults

* fix: use 1024 SIGReg vectors

* fix: require LeJEPAProjectionHead input dim

* test: drop LeJEPAProjectionHead type check

* removed lejepa from rst

* improve views documentation invariance loss

* rename to proj_dim->output_dim to follow API

* fix: harden LeJEPA loss API

* fix: use output_dim for LeJEPA head tests

* improve verification robustness

* some differents amount of local vs global views tested

* fix: use 3.8+ type hints

* sigreg

* validate projections once

* tuple instead of Tuple

* put shape validation on invariance loss