Only compile IsotropicModel.blocks

[Michal-Novomestsky]

Currently, IsotropicModel doesn't support torch.compile(). The issue seems to stem from bcs.tolist() which breaks the compiler:

DataDependentOutputException: aten._local_scalar_dense.default
While executing %item : call_method[target=item](args = (%getitem_11,))
Original traceback:
  File "src/model/walrus.py", line 301, in forward
    self.model(x, field_indices, bcs.tolist(), metadata=metadata)

Since most of the FLOPs live in self.blocks anyway (afaik), we can get most of the gains by only compiling them instead.

Note that this does not override torch.compile(model) (which will still break), only model.compile().