Replace O(N) recursive sequence_map_inverse with O(1) pack expansion

[tenpercent]

Summary

Replace the O(N) recursive sequence_map_inverse implementation with O(1) template depth using pack expansion.

Approach

• Use constexpr loop in find_source_index to locate permutation inverse indices
• Expand via pack expansion for O(1) template instantiation depth

Why It Works

Template recursion requires N template instantiations for N iterations, each with its own overhead. Constexpr loops execute within a single template instantiation, avoiding per-instantiation overhead.

Build Performance Impact

Template Instantiation Reduction (measured on device_grouped_conv3d_fwd_bias_bnorm_clamp_instance target, 248 files):

• Baseline: 7,748,880 total instantiations
• PR Replace O(N) recursive sequence_map_inverse with O(1) pack expansion #3596: 7,621,984 total instantiations
• Improvement: -126,896 instantiations (1.6% reduction)

This confirms the optimization successfully reduces template instantiation overhead by eliminating recursive template patterns in favor of pack expansion.

Test Plan

• Existing SequenceMapInverse.InverseMap and SequenceMapInverse.InverseIdentityMap tests validate correctness
• CI

Notes

• sequence_merge optimization removed from this PR (handled in Optimize sequence_gen and uniform_sequence_gen to reduce template instantiation depth #3585)
• Callers validate input via is_valid_sequence_map before calling sequence_map_inverse