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**Description**: After reviewing the prompts API, it is clear that the only way a user can explicitly mark an input variable as optional is through the `MessagePlaceholder.optional` attribute. Otherwise, the user must explicitly pass in the `input_variables` expected to be used in the `BasePromptTemplate`, which will be validated upon execution. Therefore, to semantically handle a `MessagePlaceholder` `variable_name` as optional, we will treat the `variable_name` of `MessagePlaceholder` as a `partial_variable` if it has been marked as optional. This approach aligns with how the `variable_name` of `MessagePlaceholder` is already handled [here](https://github.com/keenborder786/langchain/blob/optional_input_variables/libs/core/langchain_core/prompts/chat.py#L991). Additionally, an attribute `optional_variable` has been added to `BasePromptTemplate`, and the `variable_name` of `MessagePlaceholder` is also made part of `optional_variable` when marked as optional. Moreover, the `get_input_schema` method has been updated for `BasePromptTemplate` to differentiate between optional and non-optional variables. **Issue**: #22832, #21425 --------- Co-authored-by: Harrison Chase <hw.chase.17@gmail.com> Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>