Make this code compatible with Python 3.10.

PiperOrigin-RevId: 473457191

physics_inspired_models/utils.py

@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 """Utilities functions for Jax."""
-import collections
+from collections import abc
 import functools
 from typing import Any, Callable, Dict, Mapping, Union
 
@@ -176,7 +176,7 @@ def flatten_dict(d, parent_key: str = "", sep: str = "_") -> Dict[str, Any]:
   items = []
   for k, v in d.items():
     new_key = parent_key + sep + k if parent_key else k
-    if isinstance(v, collections.MutableMapping):
+    if isinstance(v, abc.MutableMapping):
       items.extend(flatten_dict(v, new_key, sep=sep).items())
     else:
       items.append((new_key, v))