[numpy] Fix users of NumPy APIs that are removed in NumPy 2.0.

This change migrates users of APIs removed in NumPy 2.0 to their recommended replacements (https://numpy.org/doc/stable/numpy_2_0_migration_guide.html).

This CL makes the following changes:

* `np.NAN` -> `np.nan`

PiperOrigin-RevId: 659605386

Author: hawkinsp

research/object_detection/utils/metrics.py

@@ -90,7 +90,7 @@ def compute_average_precision(precision, recall):
   if precision is None:
     if recall is not None:
       raise ValueError("If precision is None, recall must also be None")
-    return np.NAN
+    return np.nan
 
   if not isinstance(precision, np.ndarray) or not isinstance(
       recall, np.ndarray):