[refactor] loopIndexInWrite implementation moved to its root interface

[jjfumero]

Description

loopIndexInWrite implementation moved to its root interface since the implementation is identical for all device/backend types.

Problem description

n/ a.

Backend/s tested

Mark the backends affected by this PR.

• [X] OpenCL
• [X] PTX
• [X] SPIRV

OS tested

Mark the OS where this PR is tested.

• [X] Linux
• [ ] OSx
• [ ] Windows

Did you check on FPGAs?

If it is applicable, check your changes on FPGAs.

• [ ] Yes
• [X] No

How to test the new patch?

This PR affects batch processing:

tornado -ea  --jvm "-Xmx6g -Dtornado.recover.bailout=False -Dtornado.unittests.verbose=True "  -m  tornado.unittests/uk.ac.manchester.tornado.unittests.tools.TornadoTestRunner  --params "uk.ac.manchester.tornado.unittests.batches.TestBatches"

make tests