(prototype) SimCLR v2 refactor

[gabrielfruet]

SimCLR existed four times here and nothing compared the copies: the two shipped heads differed by 4,922,112 parameters, split and fused forwards gave gradients at cosine similarity 0.0837, and both examples took NTXentLoss()'s default 0.5 while the benchmark passed 0.1.

First vertical slice of the 2.0 design, SimCLR only. The other 29 methods are untouched.

• lightly/data/sample.py: a batch is a Sample of typed Views, one generic collate, plus legacy_collate.
• lightly/backbones/: Backbone and DenseBackbone protocols, a torchvision ResNet adapter, a conformance suite.
• lightly/nn/axes.py: encode(..., group_by="shape"), so BatchNorm sees 2N.
• lightly/functional/ntxent.py and lightly/optim/: the equation and the optimiser pieces get homes; old paths still import.
• examples/simclr.py and benchmarks/simclr/ replace the four old copies.

tests/test_simclr_agrees.py compares the method, not the run: blocks, view contract, fused forward, head width, and that each side states its temperature. Batch size, backbone and temperature values are free to differ, and do.

Breaking: SimCLRTransform, MoCoV1/V2Transform and DenseCLTransform return list[View]. Read views[0].data, or pass collate_fn=legacy_collate.

Testing: format-check, mypy and pytest clean bar one pre-existing DCL distributed failure; the example runs an epoch; --fast-dev-run logs train_loss and both probes; both gate mutations fail as intended.

[gabrielfruet]

Split into a stack of seven, one thing each, since this was too big to read as one diff:

1. (prototype) Add the View and Sample batch contract #2033 the View/Sample batch contract
2. (prototype) Add the backbone protocols and a torchvision ResNet adapter #2034 backbone protocols and the torchvision ResNet adapter
3. (prototype) Move the NT-Xent equation into lightly.functional #2035 the NT-Xent equation into lightly.functional
4. (prototype) Add lightly.optim with LARS, the schedulers and param_groups #2036 lightly.optim
5. (prototype) SimCLR-family transforms return labelled views #2037 SimCLR-family transforms return views
6. (prototype) One SimCLR example, one benchmark, and the gate between them #2038 the SimCLR example, benchmark and gate
7. (prototype) Delete the four old SimCLR copies #2039 delete the four old copies

The encode(..., group_by="shape") helper that was in this branch is gone from the stack. For two same-shape views it was cat and chunk behind a name, and the example exists to be read. It comes back when DINO's 224/96 multi-crop makes cat impossible.