Move callback validation to __init__

[Tushar8466]

📌 Summary

This PR fixes a bug where the @validate_callback decorator applied at
class definition time failed to validate overridden on_loop_start or
on_loop_end methods in subclasses of CallBack.

---

🐛 Problem

In the original code, @validate_callback was applied as a class-level
decorator:

@validate_callback
class Deviance(CallBack):
    ...

This means validation ran once at import time on the parent class
methods only. If a user subclassed Deviance and overrode on_loop_start,
the new method was never validated:

# This override was NEVER validated — silent bug!
class MyCallback(Deviance):
    def on_loop_start(self, gam, y, mu, some_invalid_arg):
        return 42

This could silently pass through and only crash at runtime during model
fitting, with a confusing error unrelated to the real cause.

---

✅ Fix

Moved validation into CallBack.__init__ so it runs at instance
creation time on the actual instance — including any overridden methods:

class CallBack(Core):
    def __init__(self, name=None):
        super(CallBack, self).__init__(name=name)
        validate_callback(self)  # ← runs on every instance, catches overrides

Also removed @validate_callback decorators from all subclass definitions
(Deviance, Accuracy, Diffs, Coef) since validation is now handled
automatically by the base class.

---

📂 Files Changed

File	Change
pygam/callbacks.py	Added validate_callback(self) in CallBack.__init__
pygam/callbacks.py	Removed @validate_callback from Deviance, Accuracy, Diffs, Coef

---

🔍 Before vs After

	Before	After
When validation runs	Once at import time	Every instance creation
Parent class methods validated	✅ Yes	✅ Yes
Subclass overrides validated	❌ No	✅ Yes
User-defined subclasses validated	❌ No	✅ Yes
Invalid signature caught early	❌ No	✅ Yes

---

🧪 How to Test

from pygam.callbacks import Deviance

# ✅ Valid subclass — should work fine
class GoodCallback(Deviance):
    def on_loop_start(self, gam, y, mu):
        return 99

cb = GoodCallback()  # No error


# ❌ Invalid subclass — should now raise AssertionError at instantiation
class BadCallback(Deviance):
    def on_loop_start(self, gam, y, mu, some_invalid_arg):
        return 42

cb = BadCallback()
# AssertionError: CallBack cannot reference: some_invalid_arg

---

🔗 Related Issue

Closes #[ISSUE_NUMBER]

---

📝 Notes

• The validate_callback function itself is unchanged — only when
  it is called has changed.
• The _validated flag inside validate_callback ensures there is
  no double validation if the function is somehow called twice on
  the same instance.
• This is a non-breaking change — all existing callbacks and
  correctly written subclasses will continue to work exactly as before.

[Copilot]

Pull request overview

This PR aims to ensure callback method signatures are validated on the actual overridden methods (including user subclasses), by moving callback validation from class-definition time to instance creation time.

Changes:

• Run validate_callback(self) inside CallBack.__init__ so overridden on_loop_start/on_loop_end methods are validated per instance.
• Remove @validate_callback usage from built-in callback class definitions (Deviance, Accuracy, Diffs, Coef).
• Add inline notes describing the new validation approach.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.