[BUG] 'ClassificationTemplate' object has no attribute 'explanation_template'

[Sprizgola]

Describe the bug
The variable explanation_template miss a default None value if it is not assigned.

To Reproduce
Steps to reproduce the behavior:

1. import the class: from phoenix.evals.templates import ClassificationTemplate
2. create a template without an explanation_template:

rails = ["correct", "incorrect"]
prompt_template = "prompt template ..... "
template = ClassificationTemplate(
    template= prompt_template, rails=rails
    )

3. If the code is ran, this error will pop out:
   AttributeError: 'ClassificationTemplate' object has no attribute 'explanation_template'

If the explanation_template attribute is filled, no error occurs:

rails = ["correct", "incorrect"]
prompt_template = "prompt template ..... "
template = ClassificationTemplate(
    template= prompt_template, rails=rails, 
    explanation_template="Explaination: "
    )

Expected behavior
Because the attribute is not mandatory, the class should handle the missing value.

Screenshots
Error occurs in the red rectangle because the attribute has not been set in the yellow one.

Environment (please complete the following information):

• Version v7.3.0