metric.evaluate() raise TypeError: unsupported operand type(s) for +=: 'NoneType' and 'float'

[rawheel]

Describe the bug
TypeError in metric.evaluate() when called directly.

When the evaluate() method of a metric (derived from BaseMetric) is called directly (without first calling measure()), it throws a TypeError: unsupported operand type(s) for +=: 'NoneType' and 'int/float'. This is because the evaluation_cost attribute in the BaseMetric class is initialized to None.

During the evaluation process, the code attempts to increment evaluation_cost (e.g., evaluation_cost += cost). However, since evaluation_cost is None by default, this operation results in the TypeError.

Expected Behavior: evaluation_cost should be initialized to 0 to allow numerical additions.

To Reproduce
Steps to reproduce the behavior:

1. Instantiate a metric (e.g., metric = GEval()).
2. Call metric.evaluate(test_case) directly.

Expected behavior
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Desktop (please complete the following information):

• OS: iOS
• Browser: Chrome
• Version [e.g. 22]

Additional context
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