Add docs for triton (#7719)

Author: bhavya01

docs/triton.md

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+# Custom GPU Kernels via Triton
+
+PyTorch/XLA now supports [Triton](https://openai.com/research/triton) kernels, enabling high-performance deep learning model execution on GPUs. Triton, a specialized language and compiler for GPU programming, empowers developers to write custom kernels that leverage the full potential of GPUs for various operations in deep learning models.
+
+Given a Triton kernel defined as follows:
+```python3
+@triton.jit
+def add_kernel(
+    x_ptr,  # *Pointer* to first input vector.
+    y_ptr,  # *Pointer* to second input vector.
+    output_ptr,  # *Pointer* to output vector.
+    n_elements,  # Size of the vector.
+    BLOCK_SIZE: tl.constexpr,  # Number of elements each program should process.
+    # NOTE: `constexpr` so it can be used as a shape value.
+):
+  # Triton add kernel from https://github.com/openai/triton/blob/main/python/tutorials/01-vector-add.py#L28
+  pid = tl.program_id(axis=0)
+  block_start = pid * BLOCK_SIZE
+  offsets = block_start + tl.arange(0, BLOCK_SIZE)
+  mask = offsets < n_elements
+  x = tl.load(x_ptr + offsets, mask=mask)
+  y = tl.load(y_ptr + offsets, mask=mask)
+  output = x + y
+  tl.store(output_ptr + offsets, output, mask=mask)
+
+```
+
+We can run make this kernel a part of the PyTorch/XLA execution graph as follows:
+
+```python3
+import torch
+
+import torch_xla.experimental.triton as xla_triton
+import torch_xla
+
+import triton
+import triton.language as tl
+
+size = 16
+x = torch.arange(size, dtype=torch.int64).to("xla")
+y = torch.arange(size, dtype=torch.int64).to("xla")
+output = torch.empty_like(x)
+block_size = 8
+grid = (triton.cdiv(size, block_size),)
+
+# triton_call takes the same arguments as the triton.jit function, in addition 
+to the kernel itself and the grid that is used to execute the kernel.
+All the tl.constexpr terms are passed as kwargs at the end.
+payload = xla_triton.triton_call(
+    x, y, output, size, kernel=add_kernel, grid=grid, BLOCK_SIZE=block_size)
+
+# To make the triton kernel, a part of the PyTorch/XLA graph, we create a
+# custom call node with the expected inputs, payload from triton_call,
+# the output shapes and output dtypes. The payload already contains information
+# regarding how the GPU buffers will be loaded when this node is executed.
+output = torch_xla._XLAC._xla_gpu_custom_call([x, y], payload,
+                                                [output.shape], [torch.int64])
+
+```
+
+For more complex kernels, you can also refer to the Triton Flash Attention kernel test in PyTorch/XLA.
+
+## Dependencies
+The Triton integration depends on the `triton` package to function. This code is tested with `triton==2.3.0`. To install:
+```bash
+pip install --no-deps triton==2.3.0