feat: nanoGPT implementation using Reactant

[avik-pal]

Needs

• NNlib.gather support in Reactant: feat: partial NNlib.gather support + better indexing support EnzymeAD/Reactant.jl#252
• NNlib.make_causal_mask: feat: add support for NNlib make causal mask EnzymeAD/Reactant.jl#249
• adjoint for dynamic_gather xref Jmp/scatter gather EnzymeAD/Enzyme-JAX#142

error: could not compute the adjoint for this operation %51 = "stablehlo.dynamic_gather"(%15, %0, %12) <{dimension_numbers = #stablehlo.gather<offset_dims = [0], collapsed_slice_dims = [1], start_index_map = [1], index_vector_dim = 1>}> : (tensor<64x64xf32>, tensor<64xi64>, tensor<2xi64>) -> tensor<64x64xf32>
error: could not compute the adjoint for this operation %47 = "stablehlo.dynamic_gather"(%14, %46, %12) <{dimension_numbers = #stablehlo.gather<offset_dims = [0], collapsed_slice_dims = [1], start_index_map = [1], index_vector_dim = 1>}> : (tensor<64x67xf32>, tensor<8192xi64>, tensor<2xi64>) -> tensor<64x8192xf32>
ERROR: LoadError: "failed to run pass manager on module"

[Copilot]

Copilot reviewed 3 out of 4 changed files in this pull request and generated no suggestions.

Files not reviewed (1)

• examples/NanoGPT/main.jl: Language not supported

[github-actions]

Benchmark Results (ASV)

	main	b508bb4...	main / b508bb4...
basics/overhead	0.135 ± 0.0013 μs	0.13 ± 0.0023 μs	1.04 ± 0.021
time_to_load	0.873 ± 0.0064 s	0.878 ± 0.0081 s	0.994 ± 0.012

Benchmark Plots

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.64%. Comparing base (a2167f2) to head (b508bb4).

⚠️ Current head b508bb4 differs from pull request most recent head fdd23e2

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Flag	BASE (a2167f2)	HEAD (b508bb4)
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Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1062       +/-   ##
===========================================
- Coverage   82.53%   67.64%   -14.89%     
===========================================
  Files         168      154       -14     
  Lines        6957     6408      -549     
===========================================
- Hits         5742     4335     -1407     
- Misses       1215     2073      +858     

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