Fix initiation interval of pooling and zeropadding layers on Vitis backend

[steltze]

On the Vitis backend and io_stream, zeropadding and pooling layers don't reach II=1 and are slower than for example the Conv layers

Type of change

• Bug fix (non-breaking change that fixes an issue)

Tests

Synthesized the zeropadding and pooling models in the pytests. Code achieves II=1 and Latency cycles match the tripcount

Input size = 128x128x3

C-Synthesis results with Vitis HLS 2023.2

Layer	Latency (cycles)	FFs	LUTs
Zeropadding Before	19487	169	596
Zeropadding After	17689	471	1675
Pooling Before	32769	764	1432
Pooling After	16387	795	1392

Tested also on a dummy CNN.

Test Configuration:

Checklist

• I have read the guidelines for contributing.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have installed and run pre-commit on the files I edited or added.
• I have added tests that prove my fix is effective or that my feature works.