Guard LRRangeTest and OneCycle schedulers against zero step sizes

[ebarkhordar]

Problem

Two learning-rate schedulers in deepspeed/runtime/lr_schedules.py divide by a step-size value taken directly from user config, with no validation, so a 0 step size crashes with a bare ZeroDivisionError instead of a clear configuration error:

• LRRangeTest divides the step index by self.step_size in _continuous_interval / _staircase_interval. With lr_range_test_step_size=0 the first step() raises ZeroDivisionError.
• OneCycle computes self.step_ratio = cycle_first_step_size / self.total_size in _initialize_cycle, where total_size = cycle_first_step_size + cycle_second_step_size. When both halves are 0, the constructor raises ZeroDivisionError.

Repro (CPU-only):

import torch
from deepspeed.runtime.lr_schedules import LRRangeTest, OneCycle

opt = lambda: torch.optim.SGD([torch.nn.Parameter(torch.zeros(1))], lr=0.1)

LRRangeTest(opt(), lr_range_test_step_size=0).step()          # ZeroDivisionError
OneCycle(opt(), cycle_min_lr=0.001, cycle_max_lr=0.1,
         cycle_first_step_size=0, cycle_second_step_size=0)   # ZeroDivisionError at construction

The sibling WarmupLR/WarmupCosineLR constructors already reject invalid warmup_num_steps this way (#8126, #8142, #8151); these two schedulers were skipped.

Fix

Validate at construction, before the division:

• LRRangeTest.__init__: reject a non-positive lr_range_test_step_size with a ValueError, mirroring the existing warmup_num_steps guard exactly.
• OneCycle._initialize_cycle: reject a non-positive total_size (cycle_first_step_size + cycle_second_step_size) with a ValueError.

No behavior change for valid configs: the guards only fire when the value is <= 0, which previously crashed (or, for a negative OneCycle total, produced a meaningless schedule).

Testing

Added CPU-only regression tests next to the existing scheduler-validation tests. They raise ZeroDivisionError (OneCycle) or silently accept the misconfig (LRRangeTest) on current master, and pass with this change:

pytest tests/unit/runtime/test_lr_schedulers.py -k "nonpositive or warmup_cosine or reject_invalid"
15 passed, 45 deselected

yapf, flake8, codespell clean via pre-commit run. DCO signed off.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reject zero first cycle step independently

When cycle_first_step_size=0 and cycle_second_step_size is positive, this sum-only check passes because total_size > 0, but self.step_ratio becomes 0. A get_lr() call before the first step() then enters _get_scale_factor() with x == 0 and evaluates x / self.step_ratio, raising the same ZeroDivisionError this guard is meant to prevent; DeepSpeed already exercises pre-training get_lr() via TestGetLrBeforeTrain. Please reject a zero first-step size separately, or explicitly handle a zero-length warm-up half.

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