Fix GRASP's dead similar-patient graph

[fbonc]

Problem
GRASPLayer set self.A_mat = None in __init__ and never reassigned it, so the if self.A_mat is None branch always won and the GCN was fed an identity adjacency mat. The kNN "similar-cluster" graph (which is the whole point of GRASP) was dead code.

The kNN branch also hardcoded n_neighbors=20 while the graph has only cluster_num nodes (default 2), so it would have raised n_neighbors > n_samples had it ever run. The dead-code guard prevented the crash-prone call from ever executing.

Fix
Build the kNN graph unconditionally, capping neighbors to the node count with k = min(20, cluster_num - 1), and keep identity only as a genuine fallback when there are too few clusters (k < 1). Removed the unused self.A_mat (the graph is rebuilt each forward from the current batch's cluster centers, so it can't be cached anyway).

Tests
Added a regression test that looks at the adjacency passed to the GCN. it must be a real kNN graph (off-diagonal edges, no self-loops) for cluster_num >= 2, and identity only for cluster_num == 1. Fails on the old identity-only behavior, passes on the fix.