docs: Fix installation instructions for CUDA-specific package URLs (#1893)

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## 📌 Description

We forgot to include cuda specific suffix for `flashinfer-jit-cache`
package installation instructions, this PR fixes the issue.

Also update the installation guide for nightly packages, for
flashinfer-python package:
* first install flashinfer-python nightly from custom index, without
installing dependencies (the dependencies such as torch are not
available on our self-hosted index).
* the install flashinfer-python's dependency from pypi

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Author: yzh119

README.md

@@ -58,7 +58,8 @@ pip install flashinfer-python
 **For faster initialization and offline usage**, install the optional packages to have most kernels pre-compiled:
 ```bash
 pip install flashinfer-python flashinfer-cubin
-pip install flashinfer-jit-cache --index-url https://flashinfer.ai/whl/
+# JIT cache package (replace cu129 with your CUDA version: cu128, cu129, or cu130)
+pip install flashinfer-jit-cache --index-url https://flashinfer.ai/whl/cu129
 ```
 
 This eliminates compilation and downloading overhead at runtime.
@@ -103,7 +104,8 @@ Nightly builds are available for testing the latest features:
 
 ```bash
 # Core and cubin packages
-pip install -U --pre flashinfer-python --extra-index-url https://flashinfer.ai/whl/nightly/
+pip install -U --pre flashinfer-python --index-url https://flashinfer.ai/whl/nightly/ --no-deps # Install the nightly package from custom index, without installing dependencies
+pip install flashinfer-python  # Install flashinfer-python's dependencies from PyPI
 pip install -U --pre flashinfer-cubin --index-url https://flashinfer.ai/whl/nightly/
 # JIT cache package (replace cu129 with your CUDA version: cu128, cu129, or cu130)
 pip install -U --pre flashinfer-jit-cache --index-url https://flashinfer.ai/whl/nightly/cu129

docs/installation.rst

@@ -38,7 +38,8 @@ FlashInfer provides three packages:
 .. code-block:: bash
 
     pip install flashinfer-python flashinfer-cubin
-    pip install flashinfer-jit-cache --index-url https://flashinfer.ai/whl/
+    # JIT cache package (replace cu129 with your CUDA version: cu128, cu129, or cu130)
+    pip install flashinfer-jit-cache --index-url https://flashinfer.ai/whl/cu129
 
 This eliminates compilation and downloading overhead at runtime.
 
@@ -105,7 +106,8 @@ Nightly builds are available for testing the latest features:
 .. code-block:: bash
 
     # Core and cubin packages
-    pip install -U --pre flashinfer-python --extra-index-url https://flashinfer.ai/whl/nightly/
+    pip install -U --pre flashinfer-python --index-url https://flashinfer.ai/whl/nightly/ --no-deps # Install the nightly package from custom index, without installing dependencies
+    pip install flashinfer-python  # Install flashinfer-python's dependencies from PyPI
     pip install -U --pre flashinfer-cubin --index-url https://flashinfer.ai/whl/nightly/
     # JIT cache package (replace cu129 with your CUDA version: cu128, cu129, or cu130)
     pip install -U --pre flashinfer-jit-cache --index-url https://flashinfer.ai/whl/nightly/cu129