Add lora example to GemmaCausalLM docstring (#1527)

* Add lora example to GemmaCausalLM docstring.

* Address review.

keras_nlp/models/gemma/gemma_causal_lm.py

@@ -98,6 +98,14 @@ class GemmaCausalLM(GenerativeTask):
  gemma_lm.fit(x=features, batch_size=2)
  ```
 
+ Call `fit()` with LoRA fine-tuning enabled.
+ ```python
+ features = ["The quick brown fox jumped.", "I forgot my homework."]
+ gemma_lm = keras_nlp.models.GemmaCausalLM.from_preset("gemma_2b_en")
+ gemma.backbone.enable_lora(rank=4)
+ gemma_lm.fit(x=features, batch_size=2)
+ ```
+
  Call `fit()` without preprocessing.
  ```python
  x = {