Simplify `CHGNet.predict_graph()`

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Coverage summary from Codacy

Merging #58 (35a091c147c87d8d3efef82be647685ec06f893b) into main (72608f22571b90a271212c2f7680ff4b9fda797e) - See PR on Codacy

Coverage variation	Diff coverage
Report missing for 72608f22571b90a271212c2f7680ff4b9fda797e[^1]	100.00%

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (72608f22571b90a271212c2f7680ff4b9fda797e) | Report Missing | Report Missing | Report Missing | | | Head commit (35a091c147c87d8d3efef82be647685ec06f893b) | 1170 | 883 | 75.47% | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#58) | 3 | 3 | **100.00%** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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[^1]: Codacy didn't receive coverage data for the commit, or there was an error processing the received data. Check your integration for errors and validate that your coverage setup is correct.