Bugfix - Fix DeepSpeed BF16 config validation error

[polarG]

Description
The megatron-gpt:deepspeed benchmark fails with return code 3 (INVALID_BENCHMARK_RESULT) during the BF16 training round. The benchmark runs two precision rounds (FP16 then BF16), and while FP16 succeeds, BF16 crashes at DeepSpeed initialization with:
pydantic_core._pydantic_core.ValidationError: 5 validation errors for DeepSpeedBF16Config
loss_scale - Extra inputs are not permitted
loss_scale_window - Extra inputs are not permitted
min_loss_scale - Extra inputs are not permitted
initial_scale_power - Extra inputs are not permitted
hysteresis - Extra inputs are not permitted

Root Cause
__prepare_deespeed_config() in megatron_gpt3.py uses the same precision_template for both FP16 and BF16 configs. This template includes loss-scaling parameters (loss_scale, loss_scale_window, min_loss_scale, initial_scale_power, hysteresis) that are valid for FP16 but rejected by DeepSpeedBF16Config, which uses pydantic strict validation and does not accept extra fields.

BF16 does not need loss scaling because it has sufficient dynamic range to avoid the underflow/overflow issues that FP16 faces.

This was always technically incorrect, but only became a hard failure when DeepSpeed migrated from Pydantic v1 to Pydantic v2 (around DeepSpeed v0.15–v0.16). In Pydantic v1, the extra = "forbid" setting was less strictly enforced, so the extra fields were silently ignored. Pydantic v2 strictly rejects all unknown fields with a ValidationError.

Fix
Generate precision-specific DeepSpeed configs:

• FP16: includes all loss-scaling parameters (unchanged behavior)
• BF16: only {'enabled': True}
• FP32: no precision section

This fix is backward compatible — passing only {'enabled': True} for BF16 is valid in all DeepSpeed versions, since the loss-scaling fields were never used by BF16.

[Copilot]

Pull request overview

Fixes DeepSpeed BF16 initialization failures in the Megatron-GPT benchmark by generating precision-specific DeepSpeed config sections, avoiding BF16 rejection of FP16-only loss-scaling fields under newer DeepSpeed/Pydantic validation.

Changes:

• Generate FP16 DeepSpeed config with loss-scaling parameters (unchanged behavior).
• Generate BF16 DeepSpeed config with only enabled: True to satisfy strict BF16 schema validation.
• Omit the precision section entirely for FP32 runs.

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[Copilot]

The comment says "Load deepspeed config template json file", but this function is constructing the template dict inline (no JSON template is loaded). Updating/removing this comment would avoid confusion about where the config comes from.

Suggested change

	# Load deepspeed config template json file
	# Build deepspeed config template in memory