Scan save mem rewrite masks issues with `steps=0`

[ricardoV94]

Description

import pytensor
import pytensor.tensor as pt
from pytensor.compile.mode import get_default_mode

n = pt.iscalar("n")
x0 = pt.vector("x0")
xs, _ = pytensor.scan(lambda xtm1: xtm1 + 1, outputs_info=[x0], n_steps=n)

out = xs[-1]  # Invalid when nsteps=0

fn = pytensor.function([n, x0], out)
print(fn(n=0, x0=[0, 1]))  # [1. 2.]

fn = pytensor.function([n, x0], out, mode=get_default_mode().excluding("shape_unsafe"))
print(fn(n=0, x0=[0, 1]))  # [1. 2.]

fn = pytensor.function([n, x0], out, mode=get_default_mode().excluding("scan_save_mem"))
print(fn(n=0, x0=[0, 1]))  # IndexError: index out of bounds

I suspect from this hack:

pytensor/pytensor/scan/rewriting.py

Lines 1438 to 1445 in 8454c3b

	# FIXME: This is not correct. Scan with 0 steps seems to be supported
	# Make sure the ScanSaveMem optimization never makes the new
	# number of steps to be 0 (this could happen, for instance, if
	# the optimization detects that the outputs of the Scan go through
	# subtensor nodes that end up taking no elements) because Scan with
	# 0 iterations are not supported. Make sure the new number of steps
	# is at least 1.
	nw_steps = select_max(nw_steps, 1)

But removing this hack leads to some tests failing, so other stuff may be doing wrong assumptions downstream of it (or perhaps inside the rewrite)