additional grafting config types and basic example doc

docs/optimizers.qmd

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+# Optimizers
+
+## Shampoo
+
+```yaml
+optimizer: shampoo
+optim_shampoo_betas: [0.9, 0.999]
+optim_args:
+    epsilon: 1e-12
+    max_preconditioner_dim: 8192
+    precondition_frequency: 100
+    use_decoupled_weight_decay: true
+    optim_shampoo_grafting_config_type: adam
+    optim_shampoo_grafting_config_kwargs:
+        beta2: 0.999
+        epsilon: 1e-12
+```