fix bugs in truncated_normal (#585)

* fix bugs in `truncated_normal`

* Update random.py

use `lax.nextafter` to get small values.

* Revert "Update random.py"

This reverts commit a59be4b.

* Update random.py

use `lax.nextafter` for `minval` and `maxval`

brainpy/_src/math/random.py

@@ -828,7 +828,9 @@ def truncated_normal(self, lower, upper, size=None, loc=0., scale=1., dtype=floa
     # Uniformly fill tensor with values from [l, u], then translate to
     # [2l-1, 2u-1].
     key = self.split_key() if key is None else _formalize_key(key)
-    out = jr.uniform(key, size, dtype, minval=2 * l - 1, maxval=2 * u - 1)
+    out = jr.uniform(key, size, dtype, 
+                     minval=lax.nextafter(2 * l - 1, np.array(np.inf, dtype=dtype)), 
+                     maxval=lax.nextafter(2 * u - 1, np.array(-np.inf, dtype=dtype)))
 
     # Use inverse cdf transform for normal distribution to get truncated
     # standard normal