Make op unique name generation case insensitive (#18413)

* Make op unique name generation case insensitive

Unique name generation for operations depends on checking a dict for
names currently in use. This commit makes it so that the names stored in
this dict are always lowercase so that we can check if a name already
exists regardless of the capitalization.

This helps in filesystems where file paths are case insensitive and
tensor dumps (like with tfdbg) try to follow directory structures that
correspond to the tensor names. If two tensors have names with the same
spelling, but different capitalizations, then this can lead to unintended
side effects/errors on these case-insensitive file systems.

* Change variable name to match unique_name

* Adjust op names to fix tests