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

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"