chore: rename FLASHINFER_JIT_VERBOSE to FLASHINFER_JIT_DEBUG for clarity (#1946)

<!-- .github/pull_request_template.md -->

## 📌 Description

Rename environment variable `FLASHINFER_JIT_VERBOSE` to
`FLASHINFER_JIT_DEBUG` to better reflect its actual behavior.

- `FLASHINFER_JIT_DEBUG`: Enable debug mode during compilation (disable
optimization, add debug symbols)
- The previous name `FLASHINFER_JIT_VERBOSE` implied "showing more
compilation info", which was confusing
- Maintained backward compatibility: falls back to
`FLASHINFER_JIT_VERBOSE` if `FLASHINFER_JIT_DEBUG` is not set

## 🔍 Related Issues

<!-- Link any related issues here -->

## 🚀 Pull Request Checklist

Thank you for contributing to FlashInfer! Before we review your pull
request, please make sure the following items are complete.

### ✅ Pre-commit Checks

- [x] I have installed `pre-commit` by running `pip install pre-commit`
(or used your preferred method).
- [x] I have installed the hooks with `pre-commit install`.
- [x] I have run the hooks manually with `pre-commit run --all-files`
and fixed any reported issues.

> If you are unsure about how to set up `pre-commit`, see [the
pre-commit documentation](https://pre-commit.com/).

## 🧪 Tests

- [x] Tests have been added or updated as needed.
- [ ] All tests are passing (`unittest`, etc.).

## Reviewer Notes

<!-- Optional: anything you'd like reviewers to focus on, concerns, etc.
-->


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Refactor**
* Introduced FLASHINFER_JIT_DEBUG environment variable for controlling
JIT debug builds with backward compatibility for legacy
FLASHINFER_JIT_VERBOSE.
* Enhanced debug build configuration with improved compiler and CUDA
debugging flags. Non-debug builds continue using -O3 optimizations.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: yzh119

flashinfer/jit/core.py

@@ -290,7 +290,10 @@ def gen_jit_spec(
     needs_device_linking: bool = False,
 ) -> JitSpec:
     check_cuda_arch()
-    verbose = os.environ.get("FLASHINFER_JIT_VERBOSE", "0") == "1"
+    # Use FLASHINFER_JIT_DEBUG if set, otherwise use FLASHINFER_JIT_VERBOSE (for backward compatibility)
+    debug_env = os.environ.get("FLASHINFER_JIT_DEBUG")
+    verbose_env = os.environ.get("FLASHINFER_JIT_VERBOSE", "0")
+    debug = (debug_env if debug_env is not None else verbose_env) == "1"
 
     cflags = ["-std=c++17", "-Wno-switch-bool"]
     cuda_cflags = [
@@ -302,15 +305,14 @@ def gen_jit_spec(
         "-DFLASHINFER_ENABLE_FP8_E4M3",
         "-DFLASHINFER_ENABLE_FP8_E5M2",
     ]
-    if verbose:
+    if debug:
         cflags += ["-O0", "-g"]
         cuda_cflags += [
             "-g",
             "-O0",
             "-G",
             "-lineinfo",
             "--ptxas-options=-v",
-            "--ptxas-options=--verbose,--register-usage-level=10,--warn-on-local-memory-usage",
             "-DCUTLASS_DEBUG_TRACE_LEVEL=2",
         ]
     else: