Added FusedDense layout (#19)

1. ./compute/fused_ops.go updates:
• Changed FusedDense signature to FusedDense(x, weight, bias Value,
options DenseConfig) (Value, error).
• Defined type DenseConfig struct { Activation ActivationType,
WeightLayout DenseLayout } where DenseLayout can be
DenseLayoutInputOutputs (0, default) or DenseLayoutOutputsInput
      (iota), with documentation.
• Renamed AxesLayout to AttentionAxesLayout and updated constants
(AttentionAxesLayoutBHSD, AttentionAxesLayoutBSHD) along with method
receivers.
2. Refactored AxesLayout usages: • Fixed references across ./compute,
./go-xla, ./onnx-gomlx, and ./gomlx.
3. Code Regeneration in ./compute: • Ran go generate ./... in ./compute
after updating the generators (gobackend_opsregistration and
notimplemented_generator) to recognize DenseConfig, DenseLayout, and
AttentionAxesLayout.
4. Updated gobackend Implementation: • Updated
./compute/internal/gobackend/fusedops/dense.go to handle DenseConfig and
DenseLayoutOutputsInput (contracting the last axis of weight when layout
is DenseLayoutOutputsInput).