rm fxpmath dependency

[calad0i]

Description

Remove dependency of #1139 on fxpmath, which generates a warning when pip installing for using outdated setup.cfg.

Type of change

• Housekeeping

Tests

Inherits from #1139

Checklist

• all

[calad0i]

Ok, we have two tests fails:
How many bits shall be there for -1024 and -0.03125? The current test asks for 2 for both, but technically, one can fit them in ap_fixed<1,11> and ap_fixed<1,-4>...

For the oneapi flavored ac, signed numbers (actually, all for now) needs at least 2 bits, but that is a separate issue and is addressed by #1334.

[jmitrevs]

I think I made negative values always have at least 2 bits. I figured for the use cases it was simpler with little effect. I am not opposed to changing it to be backend-specific provided it breaks nothing.

[calad0i]

By definition of the fixed point numbers, there will always be one more value on the negative side, and ac_fixed has the same behavior here. The problem is likely oneapi specific. Also, for the current oneapi to work, unsigned numbers also needs >=2 bits iirc.

[jmitrevs]

I had assumed it was just signed that required two bits, but I might be wrong. Best to confirm with @laurilaatu .

[laurilaatu]

@jmitrevs @calad0i
Currently 2 bits is required for both signed and unsigned cases for ac_fixed. For ap_fixed both cases can be 1 bit.

[jmitrevs]

Should we make 2 bits the minimum in all cases for now? This is only used for constants in threshold relu and such, so 1 vs 2 bits should be pretty insignificant.

[calad0i]

For ac_fixed it is already the case (see https://github.com/fastmachinelearning/hls4ml/blob/main/hls4ml/backends/fpga/fpga_types.py#L99). For ap_fixed there is no need to do this.