fix SOM Synapse bug

Author: rxng8

ngclearn/components/synapses/competitive/SOMSynapse.py

@@ -13,14 +13,17 @@ def _gaussian_kernel(dist, sigma): ## Gaussian neighborhood function
 def _ricker_marr_kernel(dist, sigma): ## mexican hat neighborhood function
     # p = jnp.square(dist)
     # d = sigma * sigma * 2.
-    #density = jnp.exp(-p/d) * (1. - 2./d * p)
+    # density = jnp.exp(-p/d) * (1. - 2./d * p)
     # p = jnp.square(dist/ sigma)
     # density = (1. - p) * jnp.exp(p * -0.5)
     # #return jnp.prod(density, axis=1, keepdims=True) ## calc likelihood
     # return density
     gauss_density = _gaussian_kernel(dist, sigma)
     density = gauss_density * (1. - (jnp.power(dist, 2) / (sigma ** 2)))
-    return density
+    # NOTE: Since the mexican hat density can produce negative values,
+    #   we clip to 0 to avoid this.
+    #   Negative density mess up learning
+    return jnp.maximum(density, 0.)
 
 def _euclidean_dist(a, b): ## Euclidean (L2) distance
     delta = a - b