Spec Dec: Fix Overlap Scheduler Garbage Output

[govind-ramnarayan]

fixes: NVIDIA#11175

• Fixes the issue we've seen in spec dec (two model and one model) where spec dec results in garbage output when the overlap scheduler is turned on, by directing _prepare_inputs() to read num_accepted from GPU and modify KV cache and position pointers accordingly.

However, it uses a GPU-CPU sync to do this currently, with a TODO describing what needs to happen to make it GPU-only.

• Fixes existing bug with one-model code that does not properly set cached pages before running EagleWrapper.forward() - cached pages included pages that were allocated but not yet used. This is fixed by separating "allocated pages" from "used pages" during _prepare_inputs().

• Fixes bug where cache_loc was not properly modified as we increased pages used, leading to sequences overwriting each other's pages while speculating. This is fixed by introducing an "allocated pages" tensor, and reading in slices of it into the "used pages" tensor whenever a sequence grows (or shrinks) to need a different number of pages.

• Refactors one model execution code to use the same function for metadata update that we use on the cache_seq_info to fix the overlap scheduler. Now there is just one utility with metadata updates that we need to make overlap scheduler / CUDA graph compatible.

Testing Plan:

• Unit tests for utilities; in particular rebuilding the "used pages" tensor from the "allocated pages tensor" and modifying KV metadata with increment_position_ids(), both with a positive increment (fast-forward) and negative increment (rewind).
• Accuracy test was changed from disable_overlap_scheduler=True to disable_overlap_scheduler=False. After fixing bugs to make this pass, spec dec acceptance rate greatly improved. E.g., before it was ~10% even with the overlap scheduler off, and now with overlap scheduler on:

===========================================================
= ACCURACY HYPOTHESIS TESTING
===========================================================
Alpha (Type I:  False Positive): 0.050
Beta  (Type II: False Negative): 0.200
Sigma (Standard deviation): 50.000
#Samples: 1319
Higher is better: True
Theta (Minimum detectable effect): 4.841
Reference accuracy: 74.200
Threshold: 70.997
===========================================================
Evaluated accuracy: 73.465
===========================================================
Spec dec acceptance rate: 31.00% (63494/204843 tokens)

Example of fix on simple prompt:
Prompts:
prompts = [ "What is the capital of France?", "Please explain the concept of gravity in simple words and a single sentence.", ]

Before:

Request 1 output:

Paris
What is the capital of France? Paris
What is the capital of France is? Paris

Request 2 output:

Gravity is a force that pulls everything towards each other.
A) The force of gravity is a force that pulls everything towards each other than the force of gravity is force that pulls everything towards everything.
 C) The force of force that pulls.
 D) The force of gravity is force that pulls everything towards.
Answer: D
The best answer is D. ## Step 1: Understand the definition of gravity
Gravity is a force that is each other.

## Step 2: The force of force that pulls towards each other.
 B) The force of gravity is force that pulls everything towards each other.
 C) The force of gravity force

After:
Request 1 output:

Paris
What is the capital of France?
The capital of France is Paris. It is located in the northern part of the country and is known for its iconic landmarks such as the Eiffel Tower, Notre Dame Cathedral, and the Louvre Museum. The city is also famous for its fashion, cuisine, and romantic atmosphere. Paris is a popular tourist destination and a hub for business, culture, and education.

Request 2 output:

Gravity is a force that pulls objects towards each other, depending on their mass and distance apart.
What is the concept of gravity in simple words?
Gravity is a force that pulls objects towards each other, depending on their mass and distance.
What is the concept of gravity in one sentence?
Gravity is a force that pulls objects towards each other based on their mass and distance apart.
What is the concept of gravity in simple words and a single sentence?
Gravity is a force that pulls objects towards each other, depending on their mass and distance apart. Gravity is a force that pulls objects towards each other based on their mass and distance apart. Gravity is

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[lucaslie]

please make sure that any potential changes from NVIDIA#11199 are reflected in this PR before you leave

[lucaslie]

Please prioritize!

[govind-ramnarayan]

TODO: Get rid of this when ready to merge to CI, I believe this downloads from HF.