Add VarlenNGramHashMapping for packed/varlen batching support in DeepSeek Engram

[Copilot]

Description

NGramHashMapping operates on a dense (batch_size, sequence_length) layout and mixes n-gram hash inputs across positions within a row. Packing multiple sequences into one flat buffer (as required by ragged/packed batching engines like OgaEngine) and reshaping to (1, num_tokens) would silently leak hash-mix inputs across sequence boundaries at every packed request boundary.

This adds a new com.microsoft op, VarlenNGramHashMapping, modeled on VarlenCausalConvWithState, which accepts a cumulative_sequence_length (shape (batch_size + 1), device-resident int32) offsets tensor and clamps the n-gram window — and past_ids/present_ids history — at each packed request's own boundary instead of the flat buffer's.

• Schema: New op in bert_defs.cc (registered in ms_opset.h) taking token-major packed input_ids ((total_tokens)) plus cumulative_sequence_length; past_ids/present_ids are now indexed per-request instead of per fixed-stride batch row.
• CPU kernel: Parallelizes over requests via ThreadPool::TryParallelFor; validates offsets host-side.
• CUDA kernel: Block-per-request design (mirrors VarlenCausalConvKernel); each block validates its own offsets before touching memory.
• WebGPU kernel: Workgroup-per-request dispatch for hash ids, with a separate program for present_ids.
• Prerequisite port: NGramHashMapping/EngramGate (from unmerged PR Add DeepSeek Engram contrib ops (EngramGate, NGramHashMapping) #32268) were not yet on this branch; selectively ported only the NGramHashMapping-related CPU/CUDA/WebGPU kernels, engram_helper files, and tests needed here (EngramGate and unrelated CausalConvWithState dilation/channels_last changes are out of scope).
• Docs: Hand-updated docs/ContribOperators.md and docs/OperatorKernels.md to match gen_doc output.
• Tests: Added coverage in engram_ops_test.cc for packed-vs-per-sequence equivalence, sequence-boundary no-leakage, packed chunked/decode equivalence across concurrent requests, negative ids, and invalid vocab_sizes.

# Equivalent results, but VarlenNGramHashMapping never reads across the sequence boundary
NGramHashMapping(seq_a) + NGramHashMapping(seq_b)
  == VarlenNGramHashMapping(concat(seq_a, seq_b), cumulative_sequence_length=[0, len(seq_a), len(seq_a)+len(seq_b)])

Motivation and Context

NGramHashMapping's dense (batch_size, sequence_length) layout is incompatible with OgaEngine's packed/ragged batching model (continuous batching / paged attention), where tokens from multiple in-flight requests are concatenated into a single flat, unpadded buffer. Without this change, models using DeepSeek Engram would silently corrupt n-gram hash ids at every sequence boundary whenever more than one request is batched together.

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