fix: nan error when all fparam/aparam have equal values (#5321)

For standard deviation of `fparam/aparam`, $\sigma = \sqrt{\frac{1}{N}
\sum_{i=1}^{N} (x_i - \bar{x})^2}=\sqrt{\frac{\sum x_i^2}{N} - \left(
\frac{\sum x_i}{N} \right)^2}$.
When all `fparam`/`aparam` have equal values in one dimension,
$\frac{\sum x_i^2}{N} - \left( \frac{\sum x_i}{N} \right)^2$ equals
zero.

However, it sometimes becomes a very small negative number(for example,
1e-18) due to numerical instability, so $\sqrt{\frac{\sum x_i^2}{N} -
\left( \frac{\sum x_i}{N} \right)^2}$ becomes `nan`.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Improved numerical stability in variance/std calculations by ensuring
intermediate variance values are non-negative before taking the square
root. This prevents occasional floating-point underflow from producing
invalid results and yields more reliable statistical outputs across
edge-case inputs.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: Chengqian-Zhang

deepmd/utils/env_mat_stat.py

@@ -89,8 +89,12 @@ def compute_std(self, default: float = 1e-1, protection: float = 1e-2) -> float:
         if self.number == 0:
             return default
         val = np.sqrt(
-            self.squared_sum / self.number
-            - np.multiply(self.sum / self.number, self.sum / self.number)
+            np.clip(
+                self.squared_sum / self.number
+                - np.multiply(self.sum / self.number, self.sum / self.number),
+                a_min=0,
+                a_max=None,
+            )
         )
         if np.abs(val) < protection:
             val = protection