[fbcode/caffe2] Make fmt formatter methods const (#100616)
Summary:
Staging an update to the latest fmt version triggered lots of build errors due to non-`const` methods on custom formatters. This fixes the `format()` methods to be `const` as they don't mutate any state anyway, as well as `parse()` methods that don't need to mutate internal state. This mitigates many future build errors.
Updates were identified and executed by using regular expression search/replacements such as:
`(constexpr auto parse\(ParseContext& [^)]*\)) \{` -> `$1 const {`
`(constexpr auto parse\(ParseContext& [^)]*\)) ->` -> `$1 const ->`
`(auto format\(.*, FormatContext& [^)]*\)) \{` -> `$1 const {`
`(auto format\(.*, FormatContext& [^)]*\)) ->` -> `$1 const ->`
Any changes to third-party code was then reverted. Some small changes detected from subsequent build errors were then applied.
Test Plan: CI
Differential Revision: D45463620
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100616
Approved by: https://github.com/davidberard98
