Update OG2 README convergence metrics and image, fix DATASET.md path (#1562)

Update convergence benchmarks table with final training results (train
loss 0.9444, test CE loss 0.9204, test perplexity 2.51), replace
convergence plot with updated curve (that uses the final sharded
dataset), and add missing data_files: null to DATASET.md custom sharded
parquet config example.

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Signed-off-by: Savitha Srinivasan <savithas@nvidia.com>

Author: savitha-eng

bionemo-recipes/recipes/opengenome2_llama_native_te/DATASET.md

@@ -162,6 +162,7 @@ TO 'output' (FORMAT PARQUET, PER_THREAD_OUTPUT true, FILE_SIZE_BYTES '200MB');
 ```yaml
 dataset:
   load_dataset_kwargs:
+    data_files: null
     path: "/path/to/your/resharded_parquet_dir"
     split: "train"
     streaming: true

bionemo-recipes/recipes/opengenome2_llama_native_te/README.md

@@ -26,7 +26,7 @@ which may also contribute to its lower test performance.
 
 | Model                      | Step / checkpoint | Train loss | Mean Test loss | Mean Test Perplexity |
 | -------------------------- | ----------------- | ---------- | -------------- | -------------------- |
-| LlaMA3 Recipe (OG2 7B)     | 182313            | 0.95       | 0.92           | 2.50                 |
+| LlaMA3 Recipe (OG2 7B)     | 182313            | 0.9444     | 0.9204         | 2.51                 |
 | Megatron baseline (OG2 7B) | 182313            | 1.01       | 1.019          | 2.80                 |
 
 > **Evaluation methodology:** Test losses are average NLL (negative log-likelihood) computed using