Reduce nest.map_structure and nest.pack_sequence_as time by ~20% for common

use cases (nested structures of list, tuple, dict).

Places relatively cheap type checks for list, tuple, and dict before
other more expensive checks. Specifically, this avoids calling expensive
checks like isinstance(structure, collections.abc.Mapping) and nest._is_named_tuple in the most common cases (since these abc isinstance checks take ~10x as long as normal isinstance checks).

This reduces the Python overhead of a sample 10-layer Keras Functional Model __call__ by ~5%.

PiperOrigin-RevId: 313211095
Change-Id: I227a3dc379eefef31060698d8c5be5f4bf2c1f50

tensorflow/python/util/nest.py

@@ -215,7 +215,15 @@ def _yield_sorted_items(iterable):
   Yields:
     The iterable's (key, value) pairs, in order of sorted keys.
   """
-  if isinstance(iterable, _collections_abc.Mapping):
+  # Ordered to check common structure types (list, tuple, dict) first.
+  if isinstance(iterable, list):
+    for item in enumerate(iterable):
+      yield item
+  # namedtuples handled separately to avoid expensive namedtuple check.
+  elif type(iterable) == tuple:  # pylint: disable=unidiomatic-typecheck
+    for item in enumerate(iterable):
+      yield item
+  elif isinstance(iterable, (dict, _collections_abc.Mapping)):
     # Iterate through dictionaries in a deterministic order by sorting the
     # keys. Notice this means that we ignore the original order of `OrderedDict`
     # instances. This is intentional, to avoid potential bugs caused by mixing