clustering_qr.kmeans_plusplus explicit tensors deletion

[RobertoDF]

Expand the reach of clear_cache to clustering_qr.kmeans_plusplus.
Somehow I still get OOM at

Kilosort/kilosort/clustering_qr.py

Line 202 in b2f5ded

vexp = 2 * Xg @ Xc.T - (Xc**2).sum(1)

in one particular session. With this explicit cache cleaning I manage to process the session.

The error happens with a specific Xg matrix that takes 5GB of GPU memory. I have 12GB of memory on my RTX 4070.

The explicit deletion of vexp and dexp will slow down the loop therefore happens only if the tensor is bigger than 4GB.

dexp and vexp are reassigned within each loop but somehow the GPU does not delete immediately the previous tensor. It is enough to delete the variables, it is not necessary to use directly torch.cuda.empty_cache(), that would further slow doen the loop.

Related to #746, possibly #771